[PATCH v3 7/8] gdb/aarch64: Detect vector length changes when debugging remotely

[Thiago Jung Bauermann <thiago.bauermann@linaro.org>]

If the remote target provides target description IDs in their threads
list and stop reply packets, use them to update the thread's gdbarch.
This allows debugging programs remotely that change their SVE vector
length during runtime.

GDB already supports different vector lengths in native debugging by
calling target_ops::thread_architecture, and aarch64_linux_nat_target
provides an implementation of that method.

So to provide the same feature in remote debugging, implement the
thread_architecture method in remote_target, so that the same
mechanism can be used for both the native and remote cases.  This
method returns the gdbarch corresponding to the target description
provided by the last threads list or stop reply packet.

To allow changing the architecture based on the target description,
add a new gdbarch method to allow arch-specific code to make the
adjustment.
---
 gdb/aarch64-tdep.c        | 20 ++++++++++
 gdb/arch-utils.c          |  8 ++++
 gdb/arch-utils.h          |  4 ++
 gdb/gdbarch-components.py | 15 +++++++
 gdb/gdbarch-gen.h         | 10 +++++
 gdb/gdbarch.c             | 22 ++++++++++
 gdb/remote.c              | 84 +++++++++++++++++++++++++++++++++++++++
 7 files changed, 163 insertions(+)

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index b576d3b9d99f..1e6e7116ed8a 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -3502,6 +3502,25 @@ aarch64_cannot_store_register (struct gdbarch *gdbarch, int regnum)
 	  || regnum == AARCH64_PAUTH_CMASK_REGNUM (tdep->pauth_reg_base));
 }
 
+/* Implement the "update_architecture" gdbarch method.  */
+
+static struct gdbarch *
+aarch64_update_architecture (struct gdbarch *gdbarch, const target_desc *tdesc)
+{
+  /* If this is a 32-bit architecture, then this is ARM, not AArch64.
+     There are no SVE registers here, so just return the inferior
+     architecture.  */
+  if (gdbarch_bfd_arch_info (gdbarch)->bits_per_word == 32)
+    return gdbarch;
+
+  struct gdbarch_info info;
+
+  info.bfd_arch_info = bfd_lookup_arch (bfd_arch_aarch64, bfd_mach_aarch64);
+  info.target_desc = tdesc;
+
+  return gdbarch_find_by_info (info);
+}
+
 /* Implement the stack_frame_destroyed_p gdbarch method.  */
 
 static int
@@ -3748,6 +3767,7 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   set_tdesc_pseudo_register_reggroup_p (gdbarch,
 					aarch64_pseudo_register_reggroup_p);
   set_gdbarch_cannot_store_register (gdbarch, aarch64_cannot_store_register);
+  set_gdbarch_update_architecture (gdbarch, aarch64_update_architecture);
 
   /* ABI */
   set_gdbarch_short_bit (gdbarch, 16);
diff --git a/gdb/arch-utils.c b/gdb/arch-utils.c
index 67968126488e..e8ad0aed6eb4 100644
--- a/gdb/arch-utils.c
+++ b/gdb/arch-utils.c
@@ -1098,6 +1098,14 @@ default_get_return_buf_addr (struct type *val_type, frame_info_ptr cur_frame)
   return 0;
 }
 
+/* See arch-utils.h.  */
+
+struct gdbarch *
+default_update_architecture (struct gdbarch *gdbarch, const target_desc *tdesc)
+{
+  return gdbarch;
+}
+
 /* Non-zero if we want to trace architecture code.  */
 
 #ifndef GDBARCH_DEBUG
diff --git a/gdb/arch-utils.h b/gdb/arch-utils.h
index 56690f0fd435..aeee7f51ad49 100644
--- a/gdb/arch-utils.h
+++ b/gdb/arch-utils.h
@@ -314,4 +314,8 @@ extern enum return_value_convention default_gdbarch_return_value
       struct regcache *regcache, struct value **read_value,
       const gdb_byte *writebuf);
 
+/* Default implementation of gdbarch update_architecture method.  */
+extern struct gdbarch *
+default_update_architecture (struct gdbarch *gdbarch, const target_desc *tdesc);
+
 #endif /* ARCH_UTILS_H */
diff --git a/gdb/gdbarch-components.py b/gdb/gdbarch-components.py
index 76ad2832d8a2..68b7521c9966 100644
--- a/gdb/gdbarch-components.py
+++ b/gdb/gdbarch-components.py
@@ -2744,3 +2744,18 @@ Read core file mappings
     predefault="default_read_core_file_mappings",
     invalid=False,
 )
+
+Method(
+    comment="""
+An architecture may change while the inferior is running.  For instance, the
+length of the vector registers in AArch64's Scalable Vector Extension is given
+by the contents of the VG pseudo-register.
+
+Return a gdbarch corresponding to the given target description.
+""",
+    type="struct gdbarch *",
+    name="update_architecture",
+    params=[("const target_desc *", "tdesc")],
+    predefault="default_update_architecture",
+    invalid=False,
+)
diff --git a/gdb/gdbarch-gen.h b/gdb/gdbarch-gen.h
index 32b2d96fbe01..d6068c2bc24f 100644
--- a/gdb/gdbarch-gen.h
+++ b/gdb/gdbarch-gen.h
@@ -1672,3 +1672,13 @@ extern void set_gdbarch_get_pc_address_flags (struct gdbarch *gdbarch, gdbarch_g
 typedef void (gdbarch_read_core_file_mappings_ftype) (struct gdbarch *gdbarch, struct bfd *cbfd, read_core_file_mappings_pre_loop_ftype pre_loop_cb, read_core_file_mappings_loop_ftype loop_cb);
 extern void gdbarch_read_core_file_mappings (struct gdbarch *gdbarch, struct bfd *cbfd, read_core_file_mappings_pre_loop_ftype pre_loop_cb, read_core_file_mappings_loop_ftype loop_cb);
 extern void set_gdbarch_read_core_file_mappings (struct gdbarch *gdbarch, gdbarch_read_core_file_mappings_ftype *read_core_file_mappings);
+
+/* An architecture may change while the inferior is running.  For instance, the
+   length of the vector registers in AArch64's Scalable Vector Extension is given
+   by the contents of the VG pseudo-register.
+
+   Return a gdbarch corresponding to the given target description. */
+
+typedef struct gdbarch * (gdbarch_update_architecture_ftype) (struct gdbarch *gdbarch, const target_desc *tdesc);
+extern struct gdbarch * gdbarch_update_architecture (struct gdbarch *gdbarch, const target_desc *tdesc);
+extern void set_gdbarch_update_architecture (struct gdbarch *gdbarch, gdbarch_update_architecture_ftype *update_architecture);
diff --git a/gdb/gdbarch.c b/gdb/gdbarch.c
index 46baca9c4793..07d3e060fe1f 100644
--- a/gdb/gdbarch.c
+++ b/gdb/gdbarch.c
@@ -256,6 +256,7 @@ struct gdbarch
   gdbarch_type_align_ftype *type_align = default_type_align;
   gdbarch_get_pc_address_flags_ftype *get_pc_address_flags = default_get_pc_address_flags;
   gdbarch_read_core_file_mappings_ftype *read_core_file_mappings = default_read_core_file_mappings;
+  gdbarch_update_architecture_ftype *update_architecture = default_update_architecture;
 };
 
 /* Create a new ``struct gdbarch'' based on information provided by
@@ -517,6 +518,7 @@ verify_gdbarch (struct gdbarch *gdbarch)
   /* Skip verify of type_align, invalid_p == 0 */
   /* Skip verify of get_pc_address_flags, invalid_p == 0 */
   /* Skip verify of read_core_file_mappings, invalid_p == 0 */
+  /* Skip verify of update_architecture, invalid_p == 0 */
   if (!log.empty ())
     internal_error (_("verify_gdbarch: the following are invalid ...%s"),
 		    log.c_str ());
@@ -1355,6 +1357,9 @@ gdbarch_dump (struct gdbarch *gdbarch, struct ui_file *file)
   gdb_printf (file,
 	      "gdbarch_dump: read_core_file_mappings = <%s>\n",
 	      host_address_to_string (gdbarch->read_core_file_mappings));
+  gdb_printf (file,
+	      "gdbarch_dump: update_architecture = <%s>\n",
+	      host_address_to_string (gdbarch->update_architecture));
   if (gdbarch->dump_tdep != NULL)
     gdbarch->dump_tdep (gdbarch, file);
 }
@@ -5317,3 +5322,20 @@ set_gdbarch_read_core_file_mappings (struct gdbarch *gdbarch,
 {
   gdbarch->read_core_file_mappings = read_core_file_mappings;
 }
+
+struct gdbarch *
+gdbarch_update_architecture (struct gdbarch *gdbarch, const target_desc *tdesc)
+{
+  gdb_assert (gdbarch != NULL);
+  gdb_assert (gdbarch->update_architecture != NULL);
+  if (gdbarch_debug >= 2)
+    gdb_printf (gdb_stdlog, "gdbarch_update_architecture called\n");
+  return gdbarch->update_architecture (gdbarch, tdesc);
+}
+
+void
+set_gdbarch_update_architecture (struct gdbarch *gdbarch,
+				 gdbarch_update_architecture_ftype update_architecture)
+{
+  gdbarch->update_architecture = update_architecture;
+}
diff --git a/gdb/remote.c b/gdb/remote.c
index f1d1944414c3..a17d65220408 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -506,6 +506,8 @@ class remote_target : public process_stratum_target
   gdb::byte_vector thread_info_to_thread_handle (struct thread_info *tp)
 						 override;
 
+  struct gdbarch *thread_architecture (ptid_t ptid) override;
+
   void stop (ptid_t) override;
 
   void interrupt () override;
@@ -1168,6 +1170,10 @@ struct remote_thread_info : public private_thread_info
      to stop for a watchpoint.  */
   CORE_ADDR watch_data_address = 0;
 
+  /* The architecture corresponding to the target description returned
+     in the last stop reply or threads list.  */
+  struct gdbarch *arch = nullptr;
+
   /* Get the thread's resume state.  */
   enum resume_state get_resume_state () const
   {
@@ -1186,6 +1192,8 @@ struct remote_thread_info : public private_thread_info
     m_resume_state = resume_state::RESUMED_PENDING_VCONT;
     m_resumed_pending_vcont_info.step = step;
     m_resumed_pending_vcont_info.sig = sig;
+
+    arch = nullptr;
   }
 
   /* Get the information this thread's pending vCont-resumption.
@@ -1203,6 +1211,8 @@ struct remote_thread_info : public private_thread_info
   void set_resumed ()
   {
     m_resume_state = resume_state::RESUMED;
+
+    arch = nullptr;
   }
 
 private:
@@ -4046,6 +4056,29 @@ remote_target::update_thread_list ()
 	      info->extra = std::move (item.extra);
 	      info->name = std::move (item.name);
 	      info->thread_handle = std::move (item.thread_handle);
+
+	      /* If the threads list indicated a target description for this
+		 thread, add it to the thread information.  */
+	      if (item.tdesc_id)
+		{
+		  const target_desc *tdesc
+		      = get_remote_state ()->get_tdesc (*item.tdesc_id);
+
+		  gdb_assert (tdesc != nullptr);
+
+		  /* If there's no thread-specific gdbarch, use the one from the
+		     whole inferior.  */
+		  if (info->arch == nullptr)
+		    info->arch = tp->inf->gdbarch;
+
+		  gdbarch *new_arch = gdbarch_update_architecture (info->arch,
+								   tdesc);
+		  if (new_arch != info->arch)
+		    {
+		      registers_changed_thread (tp);
+		      info->arch = new_arch;
+		    }
+		}
 	    }
 	}
     }
@@ -8152,6 +8185,36 @@ remote_target::process_stop_reply (struct stop_reply *stop_reply,
       && status->kind () != TARGET_WAITKIND_SIGNALLED
       && status->kind () != TARGET_WAITKIND_NO_RESUMED)
     {
+      thread_info *thr = find_thread_ptid (this, ptid);
+
+      /* If GDB already knows about this thread, we can give the
+	 architecture-specific code a chance to update the gdbarch based on the
+	 provided target description.  */
+      if (stop_reply->tdesc_id && thr != nullptr)
+	{
+	  const target_desc *tdesc
+	      = stop_reply->rs->get_tdesc (*stop_reply->tdesc_id);
+
+	  gdb_assert (tdesc != nullptr);
+
+	  /* If there's no gdbarch associated with the stop reply, use the one
+	     from the whole inferior.  */
+	  if (stop_reply->arch == nullptr)
+	    stop_reply->arch = thr->inf->gdbarch;
+
+	  stop_reply->arch = gdbarch_update_architecture (stop_reply->arch,
+							  tdesc);
+
+	  /* Save stop_reply->arch so that it can be returned by the
+	     thread_architecture method.  */
+	  remote_thread_info *remote_thr = get_remote_thread_info (thr);
+	  if (remote_thr->arch != stop_reply->arch)
+	    {
+	      registers_changed_thread (thr);
+	      remote_thr->arch = stop_reply->arch;
+	    }
+	}
+
       /* Expedited registers.  */
       if (!stop_reply->regcache.empty ())
 	{
@@ -14469,6 +14532,27 @@ remote_target::thread_info_to_thread_handle (struct thread_info *tp)
   return priv->thread_handle;
 }
 
+/* Implementation of the thread_architecture method for the remote target.  */
+
+struct gdbarch *
+remote_target::thread_architecture (ptid_t ptid)
+{
+  thread_info *thr = find_thread_ptid (this, ptid);
+  remote_thread_info *remote_thr;
+
+  if (thr == nullptr)
+    remote_thr = nullptr;
+  else
+    remote_thr = get_remote_thread_info (thr);
+
+  if (remote_thr == nullptr || remote_thr->arch == nullptr)
+    /* The default thread_architecture implementation is the one from
+       process_stratum_target.  */
+    return process_stratum_target::thread_architecture (ptid);
+
+  return remote_thr->arch;
+}
+
 bool
 remote_target::can_async_p ()
 {