btrace, gdbserver: remove the to_supports_btrace target method

Remove the to_supports_btrace target method and instead rely on detecting errors
when trying to enable recording.  This will also provide a suitable error
message explaining why recording is not possible.

For remote debugging, gdbserver will now always advertise branch tracing related
packets.  When talking to an older GDB, this will cause GDB to try to enable
branch tracing and gdbserver to report a suitable error message every time.

An older gdbserver will not advertise branch tracing related packets if the
one-time check failed, so a newer GDB with this patch will fail to enable branch
tracing at remote_enable_btrace() rather than at btrace_enable().  The error
message is the same in both cases so there should be no user-visible change.

gdb/
	* btrace.c (btrace_enable): Remove target_supports_btrace call.
	* nat/linux-btrace.c (perf_event_pt_event_type): Move.
	(kernel_supports_bts, kernel_supports_pt, linux_supports_bts)
	(linux_supports_pt, linux_supports_btrace): Remove.
	(linux_enable_bts): Call cpu_supports_bts.
	* nat/linux-btrace.h (linux_supports_btrace): Remove.
	* remote.c (remote_supports_btrace): Remove.
	(init_remote_ops): Remove remote_supports_btrace.
	* target-delegates.c: Regenerated.
	* target.c (target_supports_btrace): Remove.
	* target.h (target_ops) <to_supports_btrace>: Remove
	(target_supports_btrace): Remove.
	* x86-linux-nat.c (x86_linux_create_target): Remove
	linux_supports_btrace.

gdbserver/
	* linux-low.c (linux_target_ops): Remove linux_supports_btrace.
	* nto-low.c (nto_target_ops): Remove NULL for supports_btrace.
	* spu-low.c (spu_target_ops): Likewise.
	* win32-low.c (win32_target_ops): Likewise.
	* server.c (supported_btrace_packets): Report packets unconditionally.
	* target.h (target_ops) <supports_btrace>: Remove.
	(target_supports_btrace): Remove.

2 files changed, 20 insertions, 256 deletions

diff --git a/gdb/nat/linux-btrace.c b/gdb/nat/linux-btrace.c
index 2b37e41..42f56f9 100644
--- a/gdb/nat/linux-btrace.c
+++ b/gdb/nat/linux-btrace.c
@@ -175,23 +175,6 @@ perf_event_read_all (struct perf_event_buffer *pev, gdb_byte **data,
   pev->last_head = data_head;
 }
 
-/* Determine the event type.
-   Returns zero on success and fills in TYPE; returns -1 otherwise.  */
-
-static int
-perf_event_pt_event_type (int *type)
-{
-  gdb_file_up file
-    =  gdb_fopen_cloexec ("/sys/bus/event_source/devices/intel_pt/type", "r");
-  if (file == nullptr)
-    return -1;
-
-  int found = fscanf (file.get (), "%d", type);
-  if (found == 1)
-    return 0;
-  return -1;
-}
-
 /* Try to determine the start address of the Linux kernel.  */
 
 static uint64_t
@@ -376,176 +359,6 @@ perf_event_read_bts (struct btrace_target_info* tinfo, const uint8_t *begin,
   return btrace;
 }
 
-/* Check whether the kernel supports BTS.  */
-
-static int
-kernel_supports_bts (void)
-{
-  struct perf_event_attr attr;
-  pid_t child, pid;
-  int status, file;
-
-  errno = 0;
-  child = fork ();
-  switch (child)
-    {
-    case -1:
-      warning (_("test bts: cannot fork: %s."), safe_strerror (errno));
-      return 0;
-
-    case 0:
-      status = ptrace (PTRACE_TRACEME, 0, NULL, NULL);
-      if (status != 0)
-	{
-	  warning (_("test bts: cannot PTRACE_TRACEME: %s."),
-		   safe_strerror (errno));
-	  _exit (1);
-	}
-
-      status = raise (SIGTRAP);
-      if (status != 0)
-	{
-	  warning (_("test bts: cannot raise SIGTRAP: %s."),
-		   safe_strerror (errno));
-	  _exit (1);
-	}
-
-      _exit (1);
-
-    default:
-      pid = waitpid (child, &status, 0);
-      if (pid != child)
-	{
-	  warning (_("test bts: bad pid %ld, error: %s."),
-		   (long) pid, safe_strerror (errno));
-	  return 0;
-	}
-
-      if (!WIFSTOPPED (status))
-	{
-	  warning (_("test bts: expected stop. status: %d."),
-		   status);
-	  return 0;
-	}
-
-      memset (&attr, 0, sizeof (attr));
-
-      attr.type = PERF_TYPE_HARDWARE;
-      attr.config = PERF_COUNT_HW_BRANCH_INSTRUCTIONS;
-      attr.sample_period = 1;
-      attr.sample_type = PERF_SAMPLE_IP | PERF_SAMPLE_ADDR;
-      attr.exclude_kernel = 1;
-      attr.exclude_hv = 1;
-      attr.exclude_idle = 1;
-
-      file = syscall (SYS_perf_event_open, &attr, child, -1, -1, 0);
-      if (file >= 0)
-	close (file);
-
-      kill (child, SIGKILL);
-      ptrace (PTRACE_KILL, child, NULL, NULL);
-
-      pid = waitpid (child, &status, 0);
-      if (pid != child)
-	{
-	  warning (_("test bts: bad pid %ld, error: %s."),
-		   (long) pid, safe_strerror (errno));
-	  if (!WIFSIGNALED (status))
-	    warning (_("test bts: expected killed. status: %d."),
-		     status);
-	}
-
-      return (file >= 0);
-    }
-}
-
-/* Check whether the kernel supports Intel Processor Trace.  */
-
-static int
-kernel_supports_pt (void)
-{
-  struct perf_event_attr attr;
-  pid_t child, pid;
-  int status, file, type;
-
-  errno = 0;
-  child = fork ();
-  switch (child)
-    {
-    case -1:
-      warning (_("test pt: cannot fork: %s."), safe_strerror (errno));
-      return 0;
-
-    case 0:
-      status = ptrace (PTRACE_TRACEME, 0, NULL, NULL);
-      if (status != 0)
-	{
-	  warning (_("test pt: cannot PTRACE_TRACEME: %s."),
-		   safe_strerror (errno));
-	  _exit (1);
-	}
-
-      status = raise (SIGTRAP);
-      if (status != 0)
-	{
-	  warning (_("test pt: cannot raise SIGTRAP: %s."),
-		   safe_strerror (errno));
-	  _exit (1);
-	}
-
-      _exit (1);
-
-    default:
-      pid = waitpid (child, &status, 0);
-      if (pid != child)
-	{
-	  warning (_("test pt: bad pid %ld, error: %s."),
-		   (long) pid, safe_strerror (errno));
-	  return 0;
-	}
-
-      if (!WIFSTOPPED (status))
-	{
-	  warning (_("test pt: expected stop. status: %d."),
-		   status);
-	  return 0;
-	}
-
-      status = perf_event_pt_event_type (&type);
-      if (status != 0)
-	file = -1;
-      else
-	{
-	  memset (&attr, 0, sizeof (attr));
-
-	  attr.size = sizeof (attr);
-	  attr.type = type;
-	  attr.exclude_kernel = 1;
-	  attr.exclude_hv = 1;
-	  attr.exclude_idle = 1;
-
-	  file = syscall (SYS_perf_event_open, &attr, child, -1, -1, 0);
-	  if (file >= 0)
-	    close (file);
-	}
-
-      kill (child, SIGKILL);
-      ptrace (PTRACE_KILL, child, NULL, NULL);
-
-      pid = waitpid (child, &status, 0);
-      if (pid != child)
-	{
-	  warning (_("test pt: bad pid %ld, error: %s."),
-		   (long) pid, safe_strerror (errno));
-	  if (!WIFSIGNALED (status))
-	    warning (_("test pt: expected killed. status: %d."),
-		     status);
-	}
-
-      return (file >= 0);
-    }
-}
-
 /* Check whether an Intel cpu supports BTS.  */
 
 static int
@@ -596,64 +409,6 @@ cpu_supports_bts (void)
     }
 }
 
-/* Check whether the linux target supports BTS.  */
-
-static int
-linux_supports_bts (void)
-{
-  static int cached;
-
-  if (cached == 0)
-    {
-      if (!kernel_supports_bts ())
-	cached = -1;
-      else if (!cpu_supports_bts ())
-	cached = -1;
-      else
-	cached = 1;
-    }
-
-  return cached > 0;
-}
-
-/* Check whether the linux target supports Intel Processor Trace.  */
-
-static int
-linux_supports_pt (void)
-{
-  static int cached;
-
-  if (cached == 0)
-    {
-      if (!kernel_supports_pt ())
-	cached = -1;
-      else
-	cached = 1;
-    }
-
-  return cached > 0;
-}
-
-/* See linux-btrace.h.  */
-
-int
-linux_supports_btrace (struct target_ops *ops, enum btrace_format format)
-{
-  switch (format)
-    {
-    case BTRACE_FORMAT_NONE:
-      return 0;
-
-    case BTRACE_FORMAT_BTS:
-      return linux_supports_bts ();
-
-    case BTRACE_FORMAT_PT:
-      return linux_supports_pt ();
-    }
-
-  internal_error (__FILE__, __LINE__, _("Unknown branch trace format"));
-}
-
 /* Enable branch tracing in BTS format.  */
 
 static struct btrace_target_info *
@@ -664,6 +419,9 @@ linux_enable_bts (ptid_t ptid, const struct btrace_config_bts *conf)
   __u64 data_offset;
   int pid, pg;
 
+  if (!cpu_supports_bts ())
+    error (_("BTS support has been disabled for the target cpu."));
+
   gdb::unique_xmalloc_ptr<btrace_target_info> tinfo
     (XCNEW (btrace_target_info));
   tinfo->ptid = ptid;
@@ -770,6 +528,23 @@ linux_enable_bts (ptid_t ptid, const struct btrace_config_bts *conf)
 
 #if defined (PERF_ATTR_SIZE_VER5)
 
+/* Determine the event type.
+   Returns zero on success and fills in TYPE; returns -1 otherwise.  */
+
+static int
+perf_event_pt_event_type (int *type)
+{
+  gdb_file_up file =
+    gdb_fopen_cloexec ("/sys/bus/event_source/devices/intel_pt/type", "r");
+  if (file.get () == nullptr)
+    return -1;
+
+  int found = fscanf (file.get (), "%d", type);
+  if (found == 1)
+    return 0;
+  return -1;
+}
+
 /* Enable branch tracing in Intel Processor Trace format.  */
 
 static struct btrace_target_info *
@@ -1146,14 +921,6 @@ linux_btrace_conf (const struct btrace_target_info *tinfo)
 
 /* See linux-btrace.h.  */
 
-int
-linux_supports_btrace (struct target_ops *ops, enum btrace_format format)
-{
-  return 0;
-}
-
-/* See linux-btrace.h.  */
-
 struct btrace_target_info *
 linux_enable_btrace (ptid_t ptid, const struct btrace_config *conf)
 {
diff --git a/gdb/nat/linux-btrace.h b/gdb/nat/linux-btrace.h
index 31a8d9e..1180301 100644
--- a/gdb/nat/linux-btrace.h
+++ b/gdb/nat/linux-btrace.h
@@ -103,9 +103,6 @@ struct btrace_target_info
 #endif /* HAVE_LINUX_PERF_EVENT_H */
 };
 
-/* See to_supports_btrace in target.h.  */
-extern int linux_supports_btrace (struct target_ops *, enum btrace_format);
-
 /* See to_enable_btrace in target.h.  */
 extern struct btrace_target_info *
   linux_enable_btrace (ptid_t ptid, const struct btrace_config *conf);