gdb/riscv: Add read_description method for riscv_linux_nat_target

Adds riscv_linux_nat_target::read_description method to find a
suitable target description for the native linux target we are running
on.

Currently this will supply a suitably sized set of x-registers, and
will probe the kernel to see if the f-registers are readable.  If they
are readable then we currently assume that the f-registers are the
same size as the x-registers as I don't know of a good way to probe
the f-register length.  This will obviously need fixing in future.

As of Linux 4.19 there is no ptrace support for reading the
f-registers, this should appear in 4.20, so right now we only return
target descriptions without f-registers.

gdb/ChangeLog:

	* riscv-linux-nat.c: Add 'inferior.h' and 'target-descriptions.h'
	header files.
	(riscv_linux_nat_target::read_description): New method.

1 files changed, 38 insertions, 0 deletions

diff --git a/gdb/riscv-linux-nat.c b/gdb/riscv-linux-nat.c
index d51f6e3..f0705bc 100644
--- a/gdb/riscv-linux-nat.c
+++ b/gdb/riscv-linux-nat.c
@@ -21,6 +21,8 @@
 #include "gregset.h"
 #include "linux-nat.h"
 #include "riscv-tdep.h"
+#include "inferior.h"
+#include "target-descriptions.h"
 
 #include "elf/common.h"
 
@@ -34,6 +36,9 @@ public:
   /* Add our register access methods.  */
   void fetch_registers (struct regcache *regcache, int regnum) override;
   void store_registers (struct regcache *regcache, int regnum) override;
+
+  /* Read suitable target description.  */
+  const struct target_desc *read_description () override;
 };
 
 static riscv_linux_nat_target the_riscv_linux_nat_target;
@@ -155,6 +160,39 @@ fill_fpregset (const struct regcache *regcache, prfpregset_t *fpregs,
     regcache->raw_collect (RISCV_CSR_FCSR_REGNUM, &fpregs->__d.__fcsr);
 }
 
+/* Return a target description for the current target.  */
+
+const struct target_desc *
+riscv_linux_nat_target::read_description ()
+{
+  struct riscv_gdbarch_features features;
+  struct iovec iov;
+  elf_fpregset_t regs;
+  int tid;
+
+  /* Figuring out xlen is easy.  */
+  features.xlen = sizeof (elf_greg_t);
+
+  tid = inferior_ptid.lwp ();
+
+  iov.iov_base = &regs;
+  iov.iov_len = sizeof (regs);
+
+  /* Can we fetch the f-registers?  */
+  if (ptrace (PTRACE_GETREGSET, tid, NT_FPREGSET,
+	      (PTRACE_TYPE_ARG3) &iov) == -1)
+    features.flen = 0;		/* No f-registers.  */
+  else
+    {
+      /* TODO: We need a way to figure out the actual length of the
+	 f-registers.  We could have 64-bit x-registers, with 32-bit
+	 f-registers.  For now, just assumed xlen and flen match.  */
+      features.flen = features.xlen;
+    }
+
+  return riscv_create_target_description (features);
+}
+
 /* Fetch REGNUM (or all registers if REGNUM == -1) from the target
    into REGCACHE using PTRACE_GETREGSET.  */