[PATCHv5 11/11] gdb/gdbserver: share x86/linux tdesc caching

[Andrew Burgess <aburgess@redhat.com>]

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.

It is worth reading the description of the previous commit(s) to see
why this merging is not as simple as it seems, but in summary,
gdbserver used to generate a different set of possible target
descriptions than GDB.  The previous commit(s) fixed this, so now it
should be simpler to share the 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, that 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, in x86_target::low_arch_setup we could then 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 figured that adding
the x86_linux_post_init_tdesc function was good enough, so went with
that 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.

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.

This did flush out a bunch of build issues where I'd failed to add a
required file in a configure.* file, but I think everything should now
be good.
---
 gdb/Makefile.in                               |   5 +
 gdb/amd64-linux-tdep.c                        |  31 --
 gdb/arch/amd64-linux-tdesc.c                  |  61 ++++
 gdb/arch/i386-linux-tdesc.c                   |  51 +++
 gdb/arch/x86-linux-tdesc-features.c           | 247 +++++++++++++++
 .../arch/x86-linux-tdesc-features.h           |  50 +--
 gdb/arch/x86-linux-tdesc.h                    |  37 +++
 gdb/configure.nat                             |   6 +-
 gdb/configure.tgt                             |  11 +-
 gdb/i386-linux-tdep.c                         |  26 +-
 gdbserver/configure.srv                       |   9 +
 gdbserver/linux-amd64-ipa.cc                  |   2 +-
 gdbserver/linux-i386-ipa.cc                   |   2 +-
 gdbserver/linux-x86-low.cc                    |   2 +-
 gdbserver/linux-x86-tdesc.cc                  | 292 +-----------------
 15 files changed, 469 insertions(+), 363 deletions(-)
 create mode 100644 gdb/arch/amd64-linux-tdesc.c
 create mode 100644 gdb/arch/i386-linux-tdesc.c
 create mode 100644 gdb/arch/x86-linux-tdesc-features.c
 rename gdbserver/linux-x86-tdesc.h => gdb/arch/x86-linux-tdesc-features.h (52%)
 create mode 100644 gdb/arch/x86-linux-tdesc.h

diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index a24b2232daa..1e49ae396f4 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 \
@@ -1558,6 +1561,8 @@ 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 bcb9868e79e..c707745cd9a 100644
--- a/gdb/amd64-linux-tdep.c
+++ b/gdb/amd64-linux-tdep.c
@@ -1577,37 +1577,6 @@ 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 00000000000..63b5ddfcece
--- /dev/null
+++ b/gdb/arch/amd64-linux-tdesc.c
@@ -0,0 +1,61 @@
+/* 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;
+
+  xcr0 &= is_x32
+    ? x86_linux_x32_tdesc_feature_mask ()
+    : x86_linux_amd64_tdesc_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 00000000000..6d1f1793457
--- /dev/null
+++ b/gdb/arch/i386-linux-tdesc.c
@@ -0,0 +1,51 @@
+/* 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;
+
+  xcr0 &= x86_linux_i386_tdesc_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 00000000000..91abd9b1928
--- /dev/null
+++ b/gdb/arch/x86-linux-tdesc-features.c
@@ -0,0 +1,247 @@
+/* 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 cpu feature that might, or might
+   not, be checked for when creating a target description for one of i386,
+   amd64, or x32.  */
+
+struct x86_tdesc_feature {
+  /* The cpu 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 cpu features that are
+   checked when building a target description for i386, amd64, or x32.  */
+
+static constexpr x86_tdesc_feature x86_linux_all_tdesc_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 cpu features
+   that are checked for when building an i386 target description.  */
+
+static constexpr uint64_t
+x86_linux_i386_tdesc_feature_mask_1 ()
+{
+  uint64_t mask = 0;
+
+  for (const auto &entry : x86_linux_all_tdesc_features)
+    if (entry.is_i386)
+      mask |= entry.feature;
+
+  return mask;
+}
+
+/* Return a compile time constant which is a mask of all the cpu features
+   that are checked for when building an amd64 target description.  */
+
+static constexpr uint64_t
+x86_linux_amd64_tdesc_feature_mask_1 ()
+{
+  uint64_t mask = 0;
+
+  for (const auto &entry : x86_linux_all_tdesc_features)
+    if (entry.is_amd64)
+      mask |= entry.feature;
+
+  return mask;
+}
+
+/* Return a compile time constant which is a mask of all the cpu features
+   that are checked for when building an x32 target description.  */
+
+static constexpr uint64_t
+x86_linux_x32_tdesc_feature_mask_1 ()
+{
+  uint64_t mask = 0;
+
+  for (const auto &entry : x86_linux_all_tdesc_features)
+    if (entry.is_x32)
+      mask |= entry.feature;
+
+  return mask;
+}
+
+/* See arch/x86-linux-tdesc-features.h.  */
+
+uint64_t
+x86_linux_amd64_tdesc_feature_mask ()
+{
+  return x86_linux_amd64_tdesc_feature_mask_1 ();
+}
+
+/* See arch/x86-linux-tdesc-features.h.  */
+
+uint64_t
+x86_linux_x32_tdesc_feature_mask ()
+{
+  return x86_linux_x32_tdesc_feature_mask_1 ();
+}
+
+/* See arch/x86-linux-tdesc-features.h.  */
+
+uint64_t
+x86_linux_i386_tdesc_feature_mask ()
+{
+  return x86_linux_i386_tdesc_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_tdesc_features); ++i)
+    {
+      if ((xcr0 & x86_linux_all_tdesc_features[i].feature)
+	  == x86_linux_all_tdesc_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 cpu
+   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_tdesc_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 cpu
+   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_tdesc_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 cpu
+   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_tdesc_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_tdesc_features); ++i)
+    {
+      if ((idx & (1 << i)) != 0)
+	xcr0 |= x86_linux_all_tdesc_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
similarity index 52%
rename from gdbserver/linux-x86-tdesc.h
rename to gdb/arch/x86-linux-tdesc-features.h
index 3d6e0e51833..0d3db587174 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.
 
@@ -18,33 +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"
 
-extern int x86_linux_xcr0_to_tdesc_idx (uint64_t xcr0);
+/* 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.  */
 
-#ifdef IN_PROCESS_AGENT
+extern uint64_t x86_linux_amd64_tdesc_feature_mask ();
+extern uint64_t x86_linux_x32_tdesc_feature_mask ();
+extern uint64_t x86_linux_i386_tdesc_feature_mask ();
 
-/* 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.  */
+#ifdef GDBSERVER
 
-extern uint64_t x86_linux_tdesc_idx_to_xcr0 (int idx);
+/* 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);
 
-/* 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.  */
+#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);
 
 #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 00000000000..152592fcf76
--- /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 4bcc0696027..24e1824b01c 100644
--- a/gdb/configure.nat
+++ b/gdb/configure.nat
@@ -256,7 +256,8 @@ case ${gdb_host} in
 		NATDEPFILES="${NATDEPFILES} x86-nat.o nat/x86-dregs.o \
 		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/x86-linux-tdesc.o"
+		nat/x86-linux.o nat/x86-linux-dregs.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
@@ -323,7 +324,8 @@ case ${gdb_host} in
 		nat/x86-xstate.o amd64-nat.o amd64-linux-nat.o x86-linux-nat.o \
 		nat/linux-btrace.o \
 		nat/x86-linux.o nat/x86-linux-dregs.o nat/x86-linux-tdesc.o \
-		nat/amd64-linux-siginfo.o"
+		nat/amd64-linux-siginfo.o arch/x86-linux-tdesc-features.o \
+		arch/i386-linux-tdesc.o arch/amd64-linux-tdesc.o"
 		;;
 	    sparc)
 		# Host: GNU/Linux UltraSPARC
diff --git a/gdb/configure.tgt b/gdb/configure.tgt
index 47a674201f9..8326c458eb1 100644
--- a/gdb/configure.tgt
+++ b/gdb/configure.tgt
@@ -320,10 +320,13 @@ i[34567]86-*-linux*)
 	gdb_target_obs="i386-linux-tdep.o \
 			glibc-tdep.o \
 			solib-svr4.o symfile-mem.o \
-			linux-tdep.o linux-record.o"
+			linux-tdep.o linux-record.o \
+			arch/i386-linux-tdesc.o \
+			arch/x86-linux-tdesc-features.o"
 	if test "x$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 c796f87780b..d4d820b9f98 100644
--- a/gdb/i386-linux-tdep.c
+++ b/gdb/i386-linux-tdep.c
@@ -40,6 +40,7 @@
 #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"
@@ -679,29 +680,12 @@ 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 7a2702d78bf..e17b5cf280c 100644
--- a/gdbserver/configure.srv
+++ b/gdbserver/configure.srv
@@ -110,12 +110,16 @@ case "${gdbserver_host}" in
 			srv_tgtobj="${srv_tgtobj} nat/x86-linux.o"
 			srv_tgtobj="${srv_tgtobj} nat/x86-linux-dregs.o"
 			srv_tgtobj="${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="${ipa_obj} arch/i386-linux-tdesc-ipa.o"
 			;;
   i[34567]86-*-mingw*)	srv_regobj=""
 			srv_tgtobj="x86-low.o nat/x86-dregs.o win32-low.o"
@@ -374,12 +378,17 @@ case "${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 0c80812cc6f..89857f208b8 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 c1c3152fb04..8100c9f9840 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 6e23a53118b..b089e818211 100644
--- a/gdbserver/linux-x86-low.cc
+++ b/gdbserver/linux-x86-low.cc
@@ -33,6 +33,7 @@
 #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
@@ -48,7 +49,6 @@
 #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 5e12526bf17..13c80762605 100644
--- a/gdbserver/linux-x86-tdesc.cc
+++ b/gdbserver/linux-x86-tdesc.cc
@@ -17,295 +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 cpu feature that might, or might
-   not, be checked for when creating a target description for one of i386,
-   amd64, or x32.  */
-
-struct x86_tdesc_feature {
-  /* The cpu 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 cpu features that are
-   checked when building a target description for i386, amd64, or x32.  */
-
-static constexpr x86_tdesc_feature x86_linux_all_tdesc_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 cpu features
-   that are checked for when building an i386 target description.  */
-
-static constexpr uint64_t
-x86_linux_i386_tdesc_feature_mask ()
-{
-  uint64_t mask = 0;
-
-  for (const auto &entry : x86_linux_all_tdesc_features)
-    if (entry.is_i386)
-      mask |= entry.feature;
-
-  return mask;
-}
-
-/* Return a compile time constant which is a mask of all the cpu features
-   that are checked for when building an amd64 target description.  */
-
-static constexpr uint64_t
-x86_linux_amd64_tdesc_feature_mask ()
-{
-  uint64_t mask = 0;
-
-  for (const auto &entry : x86_linux_all_tdesc_features)
-    if (entry.is_amd64)
-      mask |= entry.feature;
-
-  return mask;
-}
 
-/* Return a compile time constant which is a mask of all the cpu features
-   that are checked for when building an x32 target description.  */
+/* See arch/x86-linux-tdesc.h.  */
 
-static constexpr uint64_t
-x86_linux_x32_tdesc_feature_mask ()
+void
+x86_linux_post_init_tdesc (target_desc *tdesc, bool is_64bit)
 {
-  uint64_t mask = 0;
-
-  for (const auto &entry : x86_linux_all_tdesc_features)
-    if (entry.is_x32)
-      mask |= entry.feature;
-
-  return mask;
-}
-
-/* Return a compile time constant which is a count of the number of cpu
-   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_tdesc_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 cpu
-   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_tdesc_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 cpu
-   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_tdesc_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_tdesc_features); ++i)
-    {
-     if ((xcr0 & x86_linux_all_tdesc_features[i].feature)
-	  == x86_linux_all_tdesc_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_tdesc_features); ++i)
-    {
-      if ((idx & (1 << i)) != 0)
-	xcr0 |= x86_linux_all_tdesc_features[i].feature;
-    }
-
-  return xcr0;
-}
-
-#endif /* IN_PROCESS_AGENT */
-
-#if defined __i386__ || !defined IN_PROCESS_AGENT
-
-/* A cache of all possible i386 target descriptions.  */
-
-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)
-{
-  xcr0 &= x86_linux_i386_tdesc_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_tdesc_feature_mask ();
+  if (is_64bit)
+    init_target_desc (tdesc, amd64_expedite_regs);
   else
-    xcr0 &= x86_linux_amd64_tdesc_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];
-  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);
+}
-- 
2.25.4