gdb/gdbserver: share x86/linux tdesc caching

This commit builds on the previous series of commits to share the
target description caching code between GDB and gdbserver for
x86/Linux targets.

The objective of this commit is to move the four functions (2 each of)
i386_linux_read_description and amd64_linux_read_description into the
gdb/arch/ directory and combine them so we have just a single copy of
each.  Then GDB, gdbserver, and the in-process-agent (IPA) will link
against these shared functions.

One curiosity with this patch is the function
x86_linux_post_init_tdesc.  On the gdbserver side the two functions
amd64_linux_read_description and i386_linux_read_description have some
functionality that is not present on the GDB side, there is some
additional configuration that is performed as each target description
is created, to setup the expedited registers.

To support this I've added the function x86_linux_post_init_tdesc.
This function is called from the two *_linux_read_description
functions, but is implemented separately for GDB and gdbserver.

An alternative approach that avoids adding x86_linux_post_init_tdesc
would be to have x86_linux_tdesc_for_tid return a non-const target
description, then in x86_target::low_arch_setup we could inspect the
target description to figure out if it is 64-bit or not, and modify
the target description as needed.  In the end I think that adding the
x86_linux_post_init_tdesc function is the simpler solution.

The contents of gdbserver/linux-x86-low.cc have moved to
gdb/arch/x86-linux-tdesc-features.c, and gdbserver/linux-x86-tdesc.h
has moved to gdb/arch/x86-linux-tdesc-features.h, this change leads to
some updates in the #includes in the gdbserver/ directory.

This commit also changes how target descriptions are cached.
Previously both GDB and gdbserver used static C-style arrays to act as
the tdesc cache.  This was fine, except for two problems.  Either the
C-style arrays would need to be placed in x86-linux-tdesc-features.c,
which would allow us to use the x86_linux_*_tdesc_count_1() functions
to size the arrays for us, or we'd need to hard code the array sizes
using separate #defines, which we'd then have to keep in sync with the
rest of the code in x86-linux-tdesc-features.c.

Given both of these problems I decided a better solution would be to
just switch to using a std::unordered_map to act as the cache.  This
will resize automatically, and we can use the xcr0 value as the key.

At first inspection, using xcr0 might seem to be a problem; after all
the {i386,amd64}_create_target_description functions take more than
just the xcr0 value.  However, this patch is only for x86/Linux
targets, and for x86/Linux all of the other flags passed to the tdesc
creation functions have constant values and so are irrelevant when we
consider tdesc caching.

For testing I've done the following:

  - Built on x86-64 GNU/Linux for all targets, and just for the native
    target,

  - Build on i386 GNU/Linux for all targets, and just for the native
    target,

  - Build on a 64-bit, non-x86 GNU/Linux for all targets, just for the
    native target, and for targets x86_64-*-linux and i386-*-linux.

Approved-By: Felix Willgerodt <felix.willgerodt@intel.com>

15 files changed, 499 insertions, 381 deletions

diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index 949f104..6f7f2af 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -748,6 +748,7 @@ ALL_64_TARGET_OBS = \
 	arch/aarch64-insn.o \
 	arch/aarch64-mte-linux.o \
 	arch/aarch64-scalable-linux.o \
+	arch/amd64-linux-tdesc.o \
 	arch/amd64.o \
 	arch/riscv.o \
 	bpf-tdep.o \
@@ -788,9 +789,11 @@ ALL_TARGET_OBS = \
 	arch/arm.o \
 	arch/arm-get-next-pcs.o \
 	arch/arm-linux.o \
+	arch/i386-linux-tdesc.o \
 	arch/i386.o \
 	arch/loongarch.o \
 	arch/ppc-linux-common.o \
+	arch/x86-linux-tdesc-features.o \
 	arm-bsd-tdep.o \
 	arm-fbsd-tdep.o \
 	arm-linux-tdep.o \
@@ -1559,6 +1562,8 @@ HFILES_NO_SRCDIR = \
 	arch/ppc-linux-common.h \
 	arch/ppc-linux-tdesc.h \
 	arch/riscv.h \
+	arch/x86-linux-tdesc-features.h \
+	arch/x86-linux-tdesc.h \
 	cli/cli-cmds.h \
 	cli/cli-decode.h \
 	cli/cli-script.h \
diff --git a/gdb/amd64-linux-tdep.c b/gdb/amd64-linux-tdep.c
index f9647dc..d7662ca 100644
--- a/gdb/amd64-linux-tdep.c
+++ b/gdb/amd64-linux-tdep.c
@@ -1578,37 +1578,6 @@ amd64_linux_record_signal (struct gdbarch *gdbarch,
   return 0;
 }
 
-const target_desc *
-amd64_linux_read_description (uint64_t xcr0_features_bit, bool is_x32)
-{
-  static target_desc *amd64_linux_tdescs \
-    [2/*AVX*/][2/*MPX*/][2/*AVX512*/][2/*PKRU*/] = {};
-  static target_desc *x32_linux_tdescs \
-    [2/*AVX*/][2/*AVX512*/][2/*PKRU*/] = {};
-
-  target_desc **tdesc;
-
-  if (is_x32)
-    {
-      tdesc = &x32_linux_tdescs[(xcr0_features_bit & X86_XSTATE_AVX) ? 1 : 0 ]
-	[(xcr0_features_bit & X86_XSTATE_AVX512) ? 1 : 0]
-	[(xcr0_features_bit & X86_XSTATE_PKRU) ? 1 : 0];
-    }
-  else
-    {
-      tdesc = &amd64_linux_tdescs[(xcr0_features_bit & X86_XSTATE_AVX) ? 1 : 0]
-	[(xcr0_features_bit & X86_XSTATE_MPX) ? 1 : 0]
-	[(xcr0_features_bit & X86_XSTATE_AVX512) ? 1 : 0]
-	[(xcr0_features_bit & X86_XSTATE_PKRU) ? 1 : 0];
-    }
-
-  if (*tdesc == NULL)
-    *tdesc = amd64_create_target_description (xcr0_features_bit, is_x32,
-					      true, true);
-
-  return *tdesc;
-}
-
 /* Get Linux/x86 target description from core dump.  */
 
 static const struct target_desc *
diff --git a/gdb/arch/amd64-linux-tdesc.c b/gdb/arch/amd64-linux-tdesc.c
new file mode 100644
index 0000000..e9c4a99
--- /dev/null
+++ b/gdb/arch/amd64-linux-tdesc.c
@@ -0,0 +1,66 @@
+/* Target description related code for GNU/Linux x86-64.
+
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "arch/x86-linux-tdesc.h"
+#include "arch/amd64-linux-tdesc.h"
+#include "arch/amd64.h"
+#include "arch/x86-linux-tdesc-features.h"
+
+
+/* See arch/amd64-linux-tdesc.h.  */
+
+const struct target_desc *
+amd64_linux_read_description (uint64_t xcr0, bool is_x32)
+{
+  /* The type used for the amd64 and x32 target description caches.  */
+  using tdesc_cache_type = std::unordered_map<uint64_t, const target_desc_up>;
+
+  /* Caches for the previously seen amd64 and x32 target descriptions,
+     indexed by the xcr0 value that created the target description.  These
+     need to be static within this function to ensure they are initialised
+     before first use.  */
+  static tdesc_cache_type amd64_tdesc_cache, x32_tdesc_cache;
+
+  tdesc_cache_type &tdesc_cache = is_x32 ? x32_tdesc_cache : amd64_tdesc_cache;
+
+  /* Only some bits are checked when creating a tdesc, but the XCR0 value
+     contains other feature bits that are not relevant for tdesc creation.
+     When indexing into the TDESC_CACHE we need to use a consistent xcr0
+     value otherwise we might fail to find an existing tdesc which has the
+     same set of relevant bits set.  */
+  xcr0 &= is_x32
+    ? x86_linux_x32_xcr0_feature_mask ()
+    : x86_linux_amd64_xcr0_feature_mask ();
+
+  const auto it = tdesc_cache.find (xcr0);
+  if (it != tdesc_cache.end ())
+    return it->second.get ();
+
+  /* Create the previously unseen target description.  */
+  target_desc_up tdesc (amd64_create_target_description (xcr0, is_x32,
+							 true, true));
+  x86_linux_post_init_tdesc (tdesc.get (), true);
+
+  /* Add to the cache, and return a pointer borrowed from the
+     target_desc_up.  This is safe as the cache (and the pointers contained
+     within it) are not deleted until GDB exits.  */
+  target_desc *ptr = tdesc.get ();
+  tdesc_cache.emplace (xcr0, std::move (tdesc));
+  return ptr;
+}
diff --git a/gdb/arch/i386-linux-tdesc.c b/gdb/arch/i386-linux-tdesc.c
new file mode 100644
index 0000000..9f12e59
--- /dev/null
+++ b/gdb/arch/i386-linux-tdesc.c
@@ -0,0 +1,56 @@
+/* Target description related code for GNU/Linux i386.
+
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "arch/x86-linux-tdesc.h"
+#include "arch/i386-linux-tdesc.h"
+#include "arch/i386.h"
+#include "arch/x86-linux-tdesc-features.h"
+
+/* See arch/i386-linux-tdesc.h.  */
+
+const target_desc *
+i386_linux_read_description (uint64_t xcr0)
+{
+  /* Cache of previously seen i386 target descriptions, indexed by the xcr0
+     value that created the target description.  This needs to be static
+     within this function to ensure it is initialised before first use.  */
+  static std::unordered_map<uint64_t, const target_desc_up> i386_tdesc_cache;
+
+  /* Only some bits are checked when creating a tdesc, but the XCR0 value
+     contains other feature bits that are not relevant for tdesc creation.
+     When indexing into the I386_TDESC_CACHE we need to use a consistent
+     xcr0 value otherwise we might fail to find an existing tdesc which has
+     the same set of relevant bits set.  */
+  xcr0 &= x86_linux_i386_xcr0_feature_mask ();
+
+  const auto it = i386_tdesc_cache.find (xcr0);
+  if (it != i386_tdesc_cache.end ())
+    return it->second.get ();
+
+  /* Create the previously unseen target description.  */
+  target_desc_up tdesc (i386_create_target_description (xcr0, true, false));
+  x86_linux_post_init_tdesc (tdesc.get (), false);
+
+  /* Add to the cache, and return a pointer borrowed from the
+     target_desc_up.  This is safe as the cache (and the pointers contained
+     within it) are not deleted until GDB exits.  */
+  target_desc *ptr = tdesc.get ();
+  i386_tdesc_cache.emplace (xcr0, std::move (tdesc));
+  return ptr;
+}
diff --git a/gdb/arch/x86-linux-tdesc-features.c b/gdb/arch/x86-linux-tdesc-features.c
new file mode 100644
index 0000000..ad1d919
--- /dev/null
+++ b/gdb/arch/x86-linux-tdesc-features.c
@@ -0,0 +1,267 @@
+/* Target description related code for GNU/Linux x86 (i386 and x86-64).
+
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "arch/x86-linux-tdesc-features.h"
+
+/* A structure used to describe a single xstate feature bit that might, or
+   might not, be checked for when creating a target description for one of
+   i386, amd64, or x32.
+
+   The different CPU/ABI types check for different xstate features when
+   creating a target description.
+
+   We want to cache target descriptions, and this is currently done in
+   three separate caches, one each for i386, amd64, and x32.  Additionally,
+   the caching we're discussing here is Linux only, and for Linux, the only
+   thing that has an impact on target description creation is the xcr0
+   value.
+
+   In order to ensure the cache functions correctly we need to filter out
+   only those xcr0 feature bits that are relevant, we can then cache target
+   descriptions based on the relevant feature bits.  Two xcr0 values might
+   be different, but have the same relevant feature bits.  In this case we
+   would expect the two xcr0 values to map to the same cache entry.  */
+
+struct x86_xstate_feature {
+  /* The xstate feature mask.  This is a mask against an xcr0 value.  */
+  uint64_t feature;
+
+  /* Is this feature checked when creating an i386 target description.  */
+  bool is_i386;
+
+  /* Is this feature checked when creating an amd64 target description.  */
+  bool is_amd64;
+
+  /* Is this feature checked when creating an x32 target description.  */
+  bool is_x32;
+};
+
+/* A constant table that describes all of the xstate features that are
+   checked when building a target description for i386, amd64, or x32.
+
+   If in the future, due to simplifications or refactoring, this table ever
+   ends up with 'true' for every xcr0 feature on every target type, then this
+   is an indication that this table should probably be removed, and that the
+   rest of the code in this file can be simplified.  */
+
+static constexpr x86_xstate_feature x86_linux_all_xstate_features[] = {
+  /* Feature,           i386,	amd64,	x32.  */
+  { X86_XSTATE_PKRU,	true,	true, 	true },
+  { X86_XSTATE_AVX512,	true,	true, 	true },
+  { X86_XSTATE_AVX,	true,	true, 	true },
+  { X86_XSTATE_MPX,	true,	true, 	false },
+  { X86_XSTATE_SSE,	true,	false, 	false },
+  { X86_XSTATE_X87,	true,	false, 	false }
+};
+
+/* Return a compile time constant which is a mask of all the xstate features
+   that are checked for when building an i386 target description.  */
+
+static constexpr uint64_t
+x86_linux_i386_xcr0_feature_mask_1 ()
+{
+  uint64_t mask = 0;
+
+  for (const auto &entry : x86_linux_all_xstate_features)
+    if (entry.is_i386)
+      mask |= entry.feature;
+
+  return mask;
+}
+
+/* Return a compile time constant which is a mask of all the xstate features
+   that are checked for when building an amd64 target description.  */
+
+static constexpr uint64_t
+x86_linux_amd64_xcr0_feature_mask_1 ()
+{
+  uint64_t mask = 0;
+
+  for (const auto &entry : x86_linux_all_xstate_features)
+    if (entry.is_amd64)
+      mask |= entry.feature;
+
+  return mask;
+}
+
+/* Return a compile time constant which is a mask of all the xstate features
+   that are checked for when building an x32 target description.  */
+
+static constexpr uint64_t
+x86_linux_x32_xcr0_feature_mask_1 ()
+{
+  uint64_t mask = 0;
+
+  for (const auto &entry : x86_linux_all_xstate_features)
+    if (entry.is_x32)
+      mask |= entry.feature;
+
+  return mask;
+}
+
+/* See arch/x86-linux-tdesc-features.h.  */
+
+uint64_t
+x86_linux_i386_xcr0_feature_mask ()
+{
+  return x86_linux_i386_xcr0_feature_mask_1 ();
+}
+
+/* See arch/x86-linux-tdesc-features.h.  */
+
+uint64_t
+x86_linux_amd64_xcr0_feature_mask ()
+{
+  return x86_linux_amd64_xcr0_feature_mask_1 ();
+}
+
+/* See arch/x86-linux-tdesc-features.h.  */
+
+uint64_t
+x86_linux_x32_xcr0_feature_mask ()
+{
+  return x86_linux_x32_xcr0_feature_mask_1 ();
+}
+
+#ifdef GDBSERVER
+
+/* See arch/x86-linux-tdesc-features.h.  */
+
+int
+x86_linux_xcr0_to_tdesc_idx (uint64_t xcr0)
+{
+  /* The following table shows which features are checked for when creating
+     the target descriptions (see nat/x86-linux-tdesc.c), the feature order
+     represents the bit order within the generated index number.
+
+     i386  | x87 sse mpx avx avx512 pkru
+     amd64 |         mpx avx avx512 pkru
+     i32   |             avx avx512 pkru
+
+     The features are ordered so that for each mode (i386, amd64, i32) the
+     generated index will form a continuous range.  */
+
+  int idx = 0;
+
+  for (int i = 0; i < ARRAY_SIZE (x86_linux_all_xstate_features); ++i)
+    {
+      if ((xcr0 & x86_linux_all_xstate_features[i].feature)
+	  == x86_linux_all_xstate_features[i].feature)
+	idx |= (1 << i);
+    }
+
+  return idx;
+}
+
+#endif /* GDBSERVER */
+
+#ifdef IN_PROCESS_AGENT
+
+/* Return a compile time constant which is a count of the number of xstate
+   features that are checked for when building an i386 target description.  */
+
+static constexpr int
+x86_linux_i386_tdesc_count_1 ()
+{
+  uint64_t count = 0;
+
+  for (const auto &entry : x86_linux_all_xstate_features)
+    if (entry.is_i386)
+      ++count;
+
+  gdb_assert (count > 0);
+
+  return (1 << count);
+}
+
+/* Return a compile time constant which is a count of the number of xstate
+   features that are checked for when building an amd64 target description.  */
+
+static constexpr int
+x86_linux_amd64_tdesc_count_1 ()
+{
+  uint64_t count = 0;
+
+  for (const auto &entry : x86_linux_all_xstate_features)
+    if (entry.is_amd64)
+      ++count;
+
+  gdb_assert (count > 0);
+
+  return (1 << count);
+}
+
+/* Return a compile time constant which is a count of the number of xstate
+   features that are checked for when building an x32 target description.  */
+
+static constexpr int
+x86_linux_x32_tdesc_count_1 ()
+{
+  uint64_t count = 0;
+
+  for (const auto &entry : x86_linux_all_xstate_features)
+    if (entry.is_x32)
+      ++count;
+
+  gdb_assert (count > 0);
+
+  return (1 << count);
+}
+
+/* See arch/x86-linux-tdesc-features.h.  */
+
+int
+x86_linux_amd64_tdesc_count ()
+{
+  return x86_linux_amd64_tdesc_count_1 ();
+}
+
+/* See arch/x86-linux-tdesc-features.h.  */
+
+int
+x86_linux_x32_tdesc_count ()
+{
+  return x86_linux_x32_tdesc_count_1 ();
+}
+
+/* See arch/x86-linux-tdesc-features.h.  */
+
+int
+x86_linux_i386_tdesc_count ()
+{
+  return x86_linux_i386_tdesc_count_1 ();
+}
+
+/* See arch/x86-linux-tdesc-features.h.  */
+
+uint64_t
+x86_linux_tdesc_idx_to_xcr0 (int idx)
+{
+  uint64_t xcr0 = 0;
+
+  for (int i = 0; i < ARRAY_SIZE (x86_linux_all_xstate_features); ++i)
+    {
+      if ((idx & (1 << i)) != 0)
+	xcr0 |= x86_linux_all_xstate_features[i].feature;
+    }
+
+  return xcr0;
+}
+
+#endif /* IN_PROCESS_AGENT */
diff --git a/gdbserver/linux-x86-tdesc.h b/gdb/arch/x86-linux-tdesc-features.h
index c06af21..fb0017e 100644
--- a/gdbserver/linux-x86-tdesc.h
+++ b/gdb/arch/x86-linux-tdesc-features.h
@@ -1,7 +1,6 @@
-/* Low level support for x86 (i386 and x86-64), shared between gdbserver
-   and IPA.
+/* Target description related code for GNU/Linux x86 (i386 and x86-64).
 
-   Copyright (C) 2016-2024 Free Software Foundation, Inc.
+   Copyright (C) 2024 Free Software Foundation, Inc.
 
    This file is part of GDB.
 
@@ -18,32 +17,46 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
-#ifndef GDBSERVER_LINUX_X86_TDESC_H
-#define GDBSERVER_LINUX_X86_TDESC_H
+#ifndef ARCH_X86_LINUX_TDESC_FEATURES_H
+#define ARCH_X86_LINUX_TDESC_FEATURES_H
 
-/* Convert an xcr0 value into an integer.  The integer will be passed to
-   the in-process-agent where it will then be passed to
-   x86_linux_tdesc_idx_to_xcr0 to get back the xcr0 value.  */
+#include "gdbsupport/x86-xstate.h"
+#include "gdbsupport/gdb_assert.h"
+
+/* Return a mask of X86_STATE_* feature flags.  The returned mask indicates
+   the set of features which are checked for when creating the target
+   description for each of amd64, x32, and i386.  */
+
+extern uint64_t x86_linux_amd64_xcr0_feature_mask ();
+extern uint64_t x86_linux_x32_xcr0_feature_mask ();
+extern uint64_t x86_linux_i386_xcr0_feature_mask ();
+
+#ifdef GDBSERVER
+
+/* Convert an xcr0 value into an integer.  The integer will be passed from
+   gdbserver to the in-process-agent where it will then be passed through
+   x86_linux_tdesc_idx_to_xcr0 to get back the original xcr0 value.  */
 
 extern int x86_linux_xcr0_to_tdesc_idx (uint64_t xcr0);
 
+#endif /* GDBSERVER */
+
 #ifdef IN_PROCESS_AGENT
 
+/* Return the maximum possible number of target descriptions for each of
+   amd64, x32, and i386.  These are used by the in-process-agent to
+   generate every possible target description.  */
+
+extern int x86_linux_amd64_tdesc_count ();
+extern int x86_linux_x32_tdesc_count ();
+extern int x86_linux_i386_tdesc_count ();
+
 /* Convert an index number (as returned from x86_linux_xcr0_to_tdesc_idx)
    into an xcr0 value which can then be used to create a target
    description.  */
 
 extern uint64_t x86_linux_tdesc_idx_to_xcr0 (int idx);
 
-/* Within the in-process-agent we need to pre-initialise all of the target
-   descriptions, to do this we need to know how many target descriptions
-   there are for each different target type.  These functions return the
-   target description count for the relevant target.  */
-
-extern int x86_linux_amd64_tdesc_count ();
-extern int x86_linux_x32_tdesc_count ();
-extern int x86_linux_i386_tdesc_count ();
-
 #endif /* IN_PROCESS_AGENT */
 
-#endif /* GDBSERVER_LINUX_X86_TDESC_H */
+#endif /* ARCH_X86_LINUX_TDESC_FEATURES_H */
diff --git a/gdb/arch/x86-linux-tdesc.h b/gdb/arch/x86-linux-tdesc.h
new file mode 100644
index 0000000..152592f
--- /dev/null
+++ b/gdb/arch/x86-linux-tdesc.h
@@ -0,0 +1,37 @@
+/* Target description related code for GNU/Linux x86 (i386 and x86-64).
+
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef ARCH_X86_LINUX_TDESC_H
+#define ARCH_X86_LINUX_TDESC_H
+
+struct target_desc;
+
+/* This function is called from amd64_linux_read_description and
+   i386_linux_read_description after a new target description has been
+   created, TDESC is the new target description, IS_64BIT will be true
+   when called from amd64_linux_read_description, otherwise IS_64BIT will
+   be false.  If the *_linux_read_description functions found a cached
+   target description then this function will not be called.
+
+   Both GDB and gdbserver have their own implementations of this
+   function.  */
+
+extern void x86_linux_post_init_tdesc (target_desc *tdesc, bool is_64bit);
+
+#endif /* ARCH_X86_LINUX_TDESC_H */
diff --git a/gdb/configure.nat b/gdb/configure.nat
index a30b07e..5d6669f 100644
--- a/gdb/configure.nat
+++ b/gdb/configure.nat
@@ -257,7 +257,8 @@ case ${gdb_host} in
 		nat/x86-xstate.o \
 		i386-linux-nat.o x86-linux-nat.o nat/linux-btrace.o \
 		nat/x86-linux.o nat/x86-linux-dregs.o nat/i386-linux.o \
-		nat/x86-linux-tdesc.o"
+		nat/x86-linux-tdesc.o arch/i386-linux-tdesc.o \
+		arch/x86-linux-tdesc-features.o"
 		;;
 	    ia64)
 		# Host: Intel IA-64 running GNU/Linux
@@ -324,7 +325,8 @@ case ${gdb_host} in
 		nat/x86-xstate.o amd64-nat.o amd64-linux-nat.o x86-linux-nat.o \
 		nat/linux-btrace.o \
 		nat/x86-linux.o nat/x86-linux-dregs.o nat/x86-linux-tdesc.o \
-		nat/amd64-linux-siginfo.o"
+		nat/amd64-linux-siginfo.o arch/x86-linux-tdesc-features.o \
+		arch/i386-linux-tdesc.o arch/amd64-linux-tdesc.o"
 		;;
 	    sparc)
 		# Host: GNU/Linux UltraSPARC
diff --git a/gdb/configure.tgt b/gdb/configure.tgt
index 47a6742..8326c45 100644
--- a/gdb/configure.tgt
+++ b/gdb/configure.tgt
@@ -320,10 +320,13 @@ i[34567]86-*-linux*)
 	gdb_target_obs="i386-linux-tdep.o \
 			glibc-tdep.o \
 			solib-svr4.o symfile-mem.o \
-			linux-tdep.o linux-record.o"
+			linux-tdep.o linux-record.o \
+			arch/i386-linux-tdesc.o \
+			arch/x86-linux-tdesc-features.o"
 	if test "x$enable_64_bit_bfd" = "xyes"; then
 	    # Target: GNU/Linux x86-64
-	    gdb_target_obs="amd64-linux-tdep.o ${gdb_target_obs}"
+	    gdb_target_obs="amd64-linux-tdep.o \
+			    arch/amd64-linux-tdesc.o ${gdb_target_obs}"
 	fi
 	;;
 i[34567]86-*-gnu*)
@@ -718,7 +721,9 @@ x86_64-*-linux*)
 	# Target: GNU/Linux x86-64
 	gdb_target_obs="amd64-linux-tdep.o ${i386_tobjs}  \
 			i386-linux-tdep.o glibc-tdep.o \
-			solib-svr4.o symfile-mem.o linux-tdep.o linux-record.o"
+			solib-svr4.o symfile-mem.o linux-tdep.o linux-record.o \
+			arch/i386-linux-tdesc.o arch/amd64-linux-tdesc.o \
+			arch/x86-linux-tdesc-features.o"
 	;;
 x86_64-*-freebsd* | x86_64-*-kfreebsd*-gnu)
 	# Target: FreeBSD/amd64
diff --git a/gdb/i386-linux-tdep.c b/gdb/i386-linux-tdep.c
index 6a74906..6debed2 100644
--- a/gdb/i386-linux-tdep.c
+++ b/gdb/i386-linux-tdep.c
@@ -41,6 +41,7 @@
 #include "i387-tdep.h"
 #include "gdbsupport/x86-xstate.h"
 #include "arch/i386-linux-tdesc.h"
+#include "arch/x86-linux-tdesc.h"
 
 /* The syscall's XML filename for i386.  */
 #define XML_SYSCALL_FILENAME_I386 "syscalls/i386-linux.xml"
@@ -680,29 +681,12 @@ i386_linux_core_read_x86_xsave_layout (struct gdbarch *gdbarch,
 					  layout) != 0;
 }
 
-/* See i386-linux-tdep.h.  */
+/* See arch/x86-linux-tdesc.h.  */
 
-const struct target_desc *
-i386_linux_read_description (uint64_t xcr0)
+void
+x86_linux_post_init_tdesc (target_desc *tdesc, bool is_64bit)
 {
-  if (xcr0 == 0)
-    return NULL;
-
-  static struct target_desc *i386_linux_tdescs \
-    [2/*X87*/][2/*SSE*/][2/*AVX*/][2/*MPX*/][2/*AVX512*/][2/*PKRU*/] = {};
-  struct target_desc **tdesc;
-
-  tdesc = &i386_linux_tdescs[(xcr0 & X86_XSTATE_X87) ? 1 : 0]
-    [(xcr0 & X86_XSTATE_SSE) ? 1 : 0]
-    [(xcr0 & X86_XSTATE_AVX) ? 1 : 0]
-    [(xcr0 & X86_XSTATE_MPX) ? 1 : 0]
-    [(xcr0 & X86_XSTATE_AVX512) ? 1 : 0]
-    [(xcr0 & X86_XSTATE_PKRU) ? 1 : 0];
-
-  if (*tdesc == NULL)
-    *tdesc = i386_create_target_description (xcr0, true, false);
-
-  return *tdesc;
+  /* Nothing.  */
 }
 
 /* Get Linux/x86 target description from core dump.  */
diff --git a/gdbserver/configure.srv b/gdbserver/configure.srv
index 538845b..e24e40e 100644
--- a/gdbserver/configure.srv
+++ b/gdbserver/configure.srv
@@ -111,12 +111,16 @@ case "${gdbserver_host}" in
 			srv_tgtobj="${srv_tgtobj} nat/x86-linux-dregs.o"
 			srv_tgtobj="${srv_tgtobj} nat/i386-linux.o"
 			srv_tgtobj="${srv_tgtobj} nat/x86-linux-tdesc.o"
+			srv_tgtobj="${srv_tgtobj} arch/x86-linux-tdesc-features.o"
+			srv_tgtobj="${srv_tgtobj} arch/i386-linux-tdesc.o"
 			srv_linux_usrregs=yes
 			srv_linux_regsets=yes
 			srv_linux_thread_db=yes
 			srv_linux_btrace=yes
 			ipa_obj="linux-i386-ipa.o linux-x86-tdesc-ipa.o"
 			ipa_obj="${ipa_obj} arch/i386-ipa.o"
+			ipa_obj="${ipa_obj} arch/x86-linux-tdesc-features-ipa.o"
+			ipa_obj="${ipa_obj} arch/i386-linux-tdesc-ipa.o"
 			;;
   i[34567]86-*-mingw*)	srv_regobj=""
 			srv_tgtobj="x86-low.o nat/x86-dregs.o win32-low.o"
@@ -375,12 +379,17 @@ case "${gdbserver_host}" in
 			srv_tgtobj="${srv_tgtobj} nat/x86-linux-dregs.o"
 			srv_tgtobj="${srv_tgtobj} nat/x86-linux-tdesc.o"
 			srv_tgtobj="${srv_tgtobj} nat/amd64-linux-siginfo.o"
+			srv_tgtobj="${srv_tgtobj} arch/x86-linux-tdesc-features.o"
+			srv_tgtobj="${srv_tgtobj} arch/amd64-linux-tdesc.o"
+			srv_tgtobj="${srv_tgtobj} arch/i386-linux-tdesc.o"
 			srv_linux_usrregs=yes # This is for i386 progs.
 			srv_linux_regsets=yes
 			srv_linux_thread_db=yes
 			srv_linux_btrace=yes
 			ipa_obj="linux-amd64-ipa.o linux-x86-tdesc-ipa.o"
 			ipa_obj="${ipa_obj} arch/amd64-ipa.o"
+			ipa_obj="${ipa_obj} arch/x86-linux-tdesc-features-ipa.o"
+			ipa_obj="${ipa_obj} arch/amd64-linux-tdesc-ipa.o"
 			;;
   x86_64-*-mingw*)	srv_regobj=""
 			srv_tgtobj="x86-low.o nat/x86-dregs.o"
diff --git a/gdbserver/linux-amd64-ipa.cc b/gdbserver/linux-amd64-ipa.cc
index 6f4ce6f..c53758a 100644
--- a/gdbserver/linux-amd64-ipa.cc
+++ b/gdbserver/linux-amd64-ipa.cc
@@ -20,9 +20,9 @@
 
 #include <sys/mman.h>
 #include "tracepoint.h"
-#include "linux-x86-tdesc.h"
 #include "gdbsupport/x86-xstate.h"
 #include "arch/amd64-linux-tdesc.h"
+#include "arch/x86-linux-tdesc-features.h"
 
 /* fast tracepoints collect registers.  */
 
diff --git a/gdbserver/linux-i386-ipa.cc b/gdbserver/linux-i386-ipa.cc
index 460f37e..2b36327 100644
--- a/gdbserver/linux-i386-ipa.cc
+++ b/gdbserver/linux-i386-ipa.cc
@@ -20,9 +20,9 @@
 
 #include <sys/mman.h>
 #include "tracepoint.h"
-#include "linux-x86-tdesc.h"
 #include "gdbsupport/x86-xstate.h"
 #include "arch/i386-linux-tdesc.h"
+#include "arch/x86-linux-tdesc-features.h"
 
 /* GDB register numbers.  */
 
diff --git a/gdbserver/linux-x86-low.cc b/gdbserver/linux-x86-low.cc
index 99828a8..e4a455c 100644
--- a/gdbserver/linux-x86-low.cc
+++ b/gdbserver/linux-x86-low.cc
@@ -35,6 +35,7 @@
 #endif
 
 #include "arch/i386-linux-tdesc.h"
+#include "arch/x86-linux-tdesc-features.h"
 
 #include "gdb_proc_service.h"
 /* Don't include elf/common.h if linux/elf.h got included by
@@ -50,7 +51,6 @@
 #include "nat/linux-nat.h"
 #include "nat/x86-linux.h"
 #include "nat/x86-linux-dregs.h"
-#include "linux-x86-tdesc.h"
 #include "nat/x86-linux-tdesc.h"
 
 #ifdef __x86_64__
diff --git a/gdbserver/linux-x86-tdesc.cc b/gdbserver/linux-x86-tdesc.cc
index f563689..13c8076 100644
--- a/gdbserver/linux-x86-tdesc.cc
+++ b/gdbserver/linux-x86-tdesc.cc
@@ -17,314 +17,19 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+#include "arch/x86-linux-tdesc.h"
 #include "tdesc.h"
-#include "linux-x86-tdesc.h"
-#include "arch/i386.h"
-#include "gdbsupport/x86-xstate.h"
-#ifdef __x86_64__
-#include "arch/amd64.h"
-#include "arch/amd64-linux-tdesc.h"
-#endif
 #include "x86-tdesc.h"
-#include "arch/i386-linux-tdesc.h"
-
-/* A structure used to describe a single xstate feature bit that might, or
-   might not, be checked for when creating a target description for one of
-   i386, amd64, or x32.
-
-   The different CPU/ABI types check for different xstate features when
-   creating a target description.
-
-   We want to cache target descriptions, and this is currently done in
-   three separate caches, one each for i386, amd64, and x32.  Additionally,
-   the caching we're discussing here is Linux only, and for Linux, the only
-   thing that has an impact on target description creation is the xcr0
-   value.
-
-   In order to ensure the cache functions correctly we need to filter out
-   only those xcr0 feature bits that are relevant, we can then cache target
-   descriptions based on the relevant feature bits.  Two xcr0 values might
-   be different, but have the same relevant feature bits.  In this case we
-   would expect the two xcr0 values to map to the same cache entry.  */
-
-struct x86_xstate_feature {
-  /* The xstate feature mask.  This is a mask against an xcr0 value.  */
-  uint64_t feature;
-
-  /* Is this feature checked when creating an i386 target description.  */
-  bool is_i386;
-
-  /* Is this feature checked when creating an amd64 target description.  */
-  bool is_amd64;
-
-  /* Is this feature checked when creating an x32 target description.  */
-  bool is_x32;
-};
-
-/* A constant table that describes all of the xstate features that are
-   checked when building a target description for i386, amd64, or x32.
-
-   If in the future, due to simplifications or refactoring, this table ever
-   ends up with 'true' for every xcr0 feature on every target type, then this
-   is an indication that this table should probably be removed, and that the
-   rest of the code in this file can be simplified.  */
-
-static constexpr x86_xstate_feature x86_linux_all_xstate_features[] = {
-  /* Feature,           i386,	amd64,	x32.  */
-  { X86_XSTATE_PKRU,	true,	true, 	true },
-  { X86_XSTATE_AVX512,	true,	true, 	true },
-  { X86_XSTATE_AVX,	true,	true, 	true },
-  { X86_XSTATE_MPX,	true,	true, 	false },
-  { X86_XSTATE_SSE,	true,	false, 	false },
-  { X86_XSTATE_X87,	true,	false, 	false }
-};
-
-/* Return a compile time constant which is a mask of all the xstate features
-   that are checked for when building an i386 target description.  */
-
-static constexpr uint64_t
-x86_linux_i386_xcr0_feature_mask ()
-{
-  uint64_t mask = 0;
-
-  for (const auto &entry : x86_linux_all_xstate_features)
-    if (entry.is_i386)
-      mask |= entry.feature;
-
-  return mask;
-}
-
-/* Return a compile time constant which is a mask of all the xstate features
-   that are checked for when building an amd64 target description.  */
-
-static constexpr uint64_t
-x86_linux_amd64_xcr0_feature_mask ()
-{
-  uint64_t mask = 0;
-
-  for (const auto &entry : x86_linux_all_xstate_features)
-    if (entry.is_amd64)
-      mask |= entry.feature;
-
-  return mask;
-}
-
-/* Return a compile time constant which is a mask of all the xstate features
-   that are checked for when building an x32 target description.  */
-
-static constexpr uint64_t
-x86_linux_x32_xcr0_feature_mask ()
-{
-  uint64_t mask = 0;
-
-  for (const auto &entry : x86_linux_all_xstate_features)
-    if (entry.is_x32)
-      mask |= entry.feature;
-
-  return mask;
-}
-
-/* Return a compile time constant which is a count of the number of xstate
-   features that are checked for when building an i386 target description.  */
-
-static constexpr int
-x86_linux_i386_tdesc_count_1 ()
-{
-  uint64_t count = 0;
-
-  for (const auto &entry : x86_linux_all_xstate_features)
-    if (entry.is_i386)
-      ++count;
-
-  gdb_assert (count > 0);
-
-  return (1 << count);
-}
-
-/* Return a compile time constant which is a count of the number of xstate
-   features that are checked for when building an amd64 target description.  */
-
-static constexpr int
-x86_linux_amd64_tdesc_count_1 ()
-{
-  uint64_t count = 0;
-
-  for (const auto &entry : x86_linux_all_xstate_features)
-    if (entry.is_amd64)
-      ++count;
-
-  gdb_assert (count > 0);
-
-  return (1 << count);
-}
-
-/* Return a compile time constant which is a count of the number of xstate
-   features that are checked for when building an x32 target description.  */
-
-static constexpr int
-x86_linux_x32_tdesc_count_1 ()
-{
-  uint64_t count = 0;
-
-  for (const auto &entry : x86_linux_all_xstate_features)
-    if (entry.is_x32)
-      ++count;
-
-  gdb_assert (count > 0);
-
-  return (1 << count);
-}
-
-#ifdef IN_PROCESS_AGENT
-
-/* See linux-x86-tdesc.h.  */
-
-int
-x86_linux_amd64_tdesc_count ()
-{
-  return x86_linux_amd64_tdesc_count_1 ();
-}
-
-/* See linux-x86-tdesc.h.  */
-
-int
-x86_linux_x32_tdesc_count ()
-{
-  return x86_linux_x32_tdesc_count_1 ();
-}
-
-/* See linux-x86-tdesc.h.  */
-
-int
-x86_linux_i386_tdesc_count ()
-{
-  return x86_linux_i386_tdesc_count_1 ();
-}
-
-#endif /* IN_PROCESS_AGENT */
-
-/* Convert an xcr0 value into an integer.  The integer will be passed to
-   the in-process-agent where it will then be passed to
-   x86_linux_tdesc_idx_to_xcr0 to get back the xcr0 value.  */
-
-int
-x86_linux_xcr0_to_tdesc_idx (uint64_t xcr0)
-{
-  /* The following table shows which features are checked for when creating
-     the target descriptions (see nat/x86-linux-tdesc.c), the feature order
-     represents the bit order within the generated index number.
-
-     i386  | x87 sse mpx avx avx512 pkru
-     amd64 |         mpx avx avx512 pkru
-     i32   |             avx avx512 pkru
-
-     The features are ordered so that for each mode (i386, amd64, i32) the
-     generated index will form a continuous range.  */
-
-  int idx = 0;
-
-  for (int i = 0; i < ARRAY_SIZE (x86_linux_all_xstate_features); ++i)
-    {
-     if ((xcr0 & x86_linux_all_xstate_features[i].feature)
-	  == x86_linux_all_xstate_features[i].feature)
-	idx |= (1 << i);
-    }
-
-  return idx;
-}
-
-#ifdef IN_PROCESS_AGENT
-
-/* Convert an index number (as returned from x86_linux_xcr0_to_tdesc_idx)
-   into an xcr0 value which can then be used to create a target
-   description.  */
-
-uint64_t
-x86_linux_tdesc_idx_to_xcr0 (int idx)
-{
-  uint64_t xcr0 = 0;
-
-  for (int i = 0; i < ARRAY_SIZE (x86_linux_all_xstate_features); ++i)
-    {
-      if ((idx & (1 << i)) != 0)
-	xcr0 |= x86_linux_all_xstate_features[i].feature;
-    }
-
-  return xcr0;
-}
-
-#endif /* IN_PROCESS_AGENT */
-
-#if defined __i386__ || !defined IN_PROCESS_AGENT
 
-/* A cache of all possible i386 target descriptions.  */
+/* See arch/x86-linux-tdesc.h.  */
 
-static struct target_desc *i386_tdescs[x86_linux_i386_tdesc_count_1 ()] = { };
-
-/* See nat/x86-linux-tdesc.h.  */
-
-const struct target_desc *
-i386_linux_read_description (uint64_t xcr0)
+void
+x86_linux_post_init_tdesc (target_desc *tdesc, bool is_64bit)
 {
-  xcr0 &= x86_linux_i386_xcr0_feature_mask ();
-  int idx = x86_linux_xcr0_to_tdesc_idx (xcr0);
-
-  gdb_assert (idx >= 0 && idx < x86_linux_i386_tdesc_count_1 ());
-
-  target_desc **tdesc = &i386_tdescs[idx];
-
-  if (*tdesc == nullptr)
-    {
-      *tdesc = i386_create_target_description (xcr0, true, false);
-
-      init_target_desc (*tdesc, i386_expedite_regs);
-    }
-
-  return *tdesc;
-}
-#endif
-
 #ifdef __x86_64__
-
-/* A cache of all possible amd64 target descriptions.  */
-
-static target_desc *amd64_tdescs[x86_linux_amd64_tdesc_count_1 ()] = { };
-
-/* A cache of all possible x32 target descriptions.  */
-
-static target_desc *x32_tdescs[x86_linux_x32_tdesc_count_1 ()] = { };
-
-/* See nat/x86-linux-tdesc.h.  */
-
-const struct target_desc *
-amd64_linux_read_description (uint64_t xcr0, bool is_x32)
-{
-  if (is_x32)
-    xcr0 &= x86_linux_x32_xcr0_feature_mask ();
-  else
-    xcr0 &= x86_linux_amd64_xcr0_feature_mask ();
-
-  int idx = x86_linux_xcr0_to_tdesc_idx (xcr0);
-
-  if (is_x32)
-    gdb_assert (idx >= 0 && idx < x86_linux_x32_tdesc_count_1 ());
-  else
-    gdb_assert (idx >= 0 && idx < x86_linux_amd64_tdesc_count_1 ());
-
-  target_desc **tdesc = nullptr;
-
-  if (is_x32)
-    tdesc = &x32_tdescs[idx];
+  if (is_64bit)
+    init_target_desc (tdesc, amd64_expedite_regs);
   else
-    tdesc = &amd64_tdescs[idx];
-
-  if (*tdesc == nullptr)
-    {
-      *tdesc = amd64_create_target_description (xcr0, is_x32, true, true);
-
-      init_target_desc (*tdesc, amd64_expedite_regs);
-    }
-  return *tdesc;
-}
-
 #endif
+    init_target_desc (tdesc, i386_expedite_regs);
+}