[PATCH 1/2] gdb/aarch64: Fix thread's gdbarch when SVE vector length changes

[Thiago Jung Bauermann via Gdb-patches <gdb-patches@sourceware.org>]

When the inferior program changes the SVE length, GDB can stop tracking
some registers as it obtains the new gdbarch that corresponds to the
updated length:

  Breakpoint 1, do_sve_ioctl_test () at sve-ioctls.c:44
  44              res = prctl(PR_SVE_SET_VL, i, 0, 0, 0, 0);
  (gdb) print i
  $2 = 32
  (gdb) info registers
          ⋮
  [ snip registers x0 to x30 ]
          ⋮
  sp             0xffffffffeff0      0xffffffffeff0
  pc             0xaaaaaaaaa8ac      0xaaaaaaaaa8ac <do_sve_ioctl_test+112>
  cpsr           0x60000000          [ EL=0 BTYPE=0 C Z ]
  fpsr           0x0                 0
  fpcr           0x0                 0
  vg             0x8                 8
  tpidr          0xfffff7fcb320      0xfffff7fcb320
  (gdb) next
  45              if (res < 0) {
  (gdb) info registers
          ⋮
  [ snip registers x0 to x30 ]
          ⋮
  sp             0xffffffffeff0      0xffffffffeff0
  pc             0xaaaaaaaaa8cc      0xaaaaaaaaa8cc <do_sve_ioctl_test+144>
  cpsr           0x200000            [ EL=0 BTYPE=0 SS ]
  fpsr           0x0                 0
  fpcr           0x0                 0
  vg             0x4                 4
  (gdb)

Notice that register tpidr disappeared when vg (which holds the vector
length) changed from 8 to 4.  The tpidr register is provided by the
org.gnu.gdb.aarch64.tls feature.

This happens because the code that searches for a new gdbarch to match the
new vector length in aarch64_linux_nat_target::thread_architecture doesn't
take into account the features present in the target description associated
with the previous gdbarch.  This patch makes it do that.
---
 gdb/aarch64-linux-nat.c | 11 ++++++++---
 gdb/aarch64-tdep.c      | 25 +++++++++++++++++++++++++
 gdb/aarch64-tdep.h      |  2 ++
 3 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/gdb/aarch64-linux-nat.c b/gdb/aarch64-linux-nat.c
index a457fcd48ad8..5963e246b43f 100644
--- a/gdb/aarch64-linux-nat.c
+++ b/gdb/aarch64-linux-nat.c
@@ -900,11 +900,16 @@ aarch64_linux_nat_target::thread_architecture (ptid_t ptid)
 
   /* We reach here if the vector length for the thread is different from its
      value at process start.  Lookup gdbarch via info (potentially creating a
-     new one), stashing the vector length inside id.  Use -1 for when SVE
-     unavailable, to distinguish from an unset value of 0.  */
+     new one) by using a target description that corresponds to the new vq value
+     and the current architecture features.  */
+
+  const struct target_desc *tdesc = gdbarch_target_desc (inf->gdbarch);
+  aarch64_features features = aarch64_features_from_target_desc (tdesc);
+  features.vq = vq;
+
   struct gdbarch_info info;
   info.bfd_arch_info = bfd_lookup_arch (bfd_arch_aarch64, bfd_mach_aarch64);
-  info.id = (int *) (vq == 0 ? -1 : vq);
+  info.target_desc = aarch64_create_target_description (features);
   return gdbarch_find_by_info (info);
 }
 
diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 8670197a8889..8b89b877f8f0 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -3352,6 +3352,31 @@ aarch64_read_description (const aarch64_features &features)
   return tdesc;
 }
 
+/* Get the AArch64 features present in the given target description. */
+
+aarch64_features
+aarch64_features_from_target_desc (const struct target_desc *tdesc)
+{
+  aarch64_features features;
+  const struct tdesc_feature *feature;
+
+  if (tdesc == nullptr)
+    return features;
+
+  feature = tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.sve");
+  if (feature != nullptr)
+    features.vq = tdesc_register_bitsize (feature, "z0") / 128;
+
+  features.pauth
+      = (tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.pauth") != nullptr);
+  features.mte
+      = (tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.mte") != nullptr);
+  features.tls
+      = (tdesc_find_feature (tdesc, "org.gnu.gdb.aarch64.tls") != nullptr);
+
+  return features;
+}
+
 /* Return the VQ used when creating the target description TDESC.  */
 
 static uint64_t
diff --git a/gdb/aarch64-tdep.h b/gdb/aarch64-tdep.h
index 5bdd733dce32..d8513023c376 100644
--- a/gdb/aarch64-tdep.h
+++ b/gdb/aarch64-tdep.h
@@ -121,6 +121,8 @@ struct aarch64_gdbarch_tdep : gdbarch_tdep_base
 };
 
 const target_desc *aarch64_read_description (const aarch64_features &features);
+aarch64_features
+aarch64_features_from_target_desc (const struct target_desc *tdesc);
 
 extern int aarch64_process_record (struct gdbarch *gdbarch,
 			       struct regcache *regcache, CORE_ADDR addr);