[PATCH 8/8] gdb/aarch64: When remote debugging detect changes in the target description

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

If the remote target provides an expedited VG register, use it to update
its target description.  This allows debugging programs that change their
SVE vector length during runtime.

This is accomplished by implementing the thread_architecture method in
remote_target, which returns the gdbarch corresponding to the expedited
registers provided by the last stop reply.

To allow changing the architecture based on the expedited registers, add a
new gdbarch method to allow arch-specific code to make the adjustment.
---
 gdb/aarch64-tdep.c        | 28 +++++++++++++++++++++++++-
 gdb/arch-utils.c          |  9 +++++++++
 gdb/arch-utils.h          |  5 +++++
 gdb/gdbarch-components.py | 16 +++++++++++++++
 gdb/gdbarch-gen.h         | 11 ++++++++++
 gdb/gdbarch.c             | 23 +++++++++++++++++++++
 gdb/remote.c              | 42 +++++++++++++++++++++++++++++++++++++++
 7 files changed, 133 insertions(+), 1 deletion(-)

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 18c2b1ec1523..acfd84ea3f33 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -3420,7 +3420,8 @@ aarch64_cannot_store_register (struct gdbarch *gdbarch, int regnum)
 	  || regnum == AARCH64_PAUTH_CMASK_REGNUM (tdep->pauth_reg_base));
 }
 
-/* Helper function for the "thread_architecture" target_ops method.
+/* Helper function for both the "update_architecture" gdbarch method and the
+   "thread_architecture" target_ops method.
 
    Returns a new gdbarch that is equivalent to the given gdbarch, but with SVE
    registers reflecting the given vq value.  */
@@ -3454,6 +3455,30 @@ aarch64_update_gdbarch (struct gdbarch *gdbarch, uint64_t vq)
   return gdbarch_find_by_info (info);
 }
 
+/* Implement the "update_architecture" gdbarch method.  */
+
+static struct gdbarch *
+aarch64_update_architecture (struct gdbarch *gdbarch,
+			     const std::vector<cached_reg_t> &regcache)
+{
+  gdb_assert (gdbarch != NULL);
+
+  /* Look for the VG register.  */
+  auto it = find_if (regcache.cbegin (), regcache.cend (),
+		     [] (const cached_reg_t &reg) {
+		       return reg.num == AARCH64_SVE_VG_REGNUM;
+		     });
+
+  /* No VG register was provided.  Don't change gdbarch.  */
+  if (it == regcache.cend ())
+    return gdbarch;
+
+  ULONGEST vg = extract_unsigned_integer (it->data, 8,
+					  gdbarch_byte_order (gdbarch));
+
+  return aarch64_update_gdbarch (gdbarch, sve_vq_from_vg (vg));
+}
+
 /* Implement the stack_frame_destroyed_p gdbarch method.  */
 
 static int
@@ -3667,6 +3692,7 @@ 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 9bd4f0ddae66..1c75d100b55b 100644
--- a/gdb/arch-utils.c
+++ b/gdb/arch-utils.c
@@ -1092,6 +1092,15 @@ default_read_core_file_mappings
 {
 }
 
+/* See arch-utils.h.  */
+
+struct gdbarch *
+default_update_architecture (struct gdbarch *gdbarch,
+			     const std::vector<cached_reg_t> &regcache)
+{
+  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 f850e5fd6e78..f81e0ad78fa3 100644
--- a/gdb/arch-utils.h
+++ b/gdb/arch-utils.h
@@ -300,4 +300,9 @@ extern void default_read_core_file_mappings
    struct bfd *cbfd,
    read_core_file_mappings_pre_loop_ftype pre_loop_cb,
    read_core_file_mappings_loop_ftype loop_cb);
+
+/* Default implementation of gdbarch update_architecture method.  */
+extern struct gdbarch *
+default_update_architecture (struct gdbarch *gdbarch,
+			     const std::vector<cached_reg_t> &regcache);
 #endif /* ARCH_UTILS_H */
diff --git a/gdb/gdbarch-components.py b/gdb/gdbarch-components.py
index 71aa5991fbe0..5a23b2ac48b7 100644
--- a/gdb/gdbarch-components.py
+++ b/gdb/gdbarch-components.py
@@ -2652,3 +2652,19 @@ Read core file mappings
     predefault="default_read_core_file_mappings",
     invalid=False,
 )
+
+Method(
+    comment="""
+An architecture may change based on the current contents of its registers.
+For instance, the length of the vector registers in AArch64's Scalable Vector
+Extension is given by the contents of the VL pseudo-register.
+
+This method allows an architecture to provide a new gdbarch corresponding to
+the registers in the given regcache.
+""",
+    type="struct gdbarch *",
+    name="update_architecture",
+    params=[("const std::vector<cached_reg_t> &", "regcache")],
+    predefault="default_update_architecture",
+    invalid=False,
+)
diff --git a/gdb/gdbarch-gen.h b/gdb/gdbarch-gen.h
index 0504962e50d2..b543a9878930 100644
--- a/gdb/gdbarch-gen.h
+++ b/gdb/gdbarch-gen.h
@@ -1627,3 +1627,14 @@ 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 based on the current contents of its registers.
+   For instance, the length of the vector registers in AArch64's Scalable Vector
+   Extension is given by the contents of the VL pseudo-register.
+
+   This method allows an architecture to provide a new gdbarch corresponding to
+   the registers in the given regcache. */
+
+typedef struct gdbarch * (gdbarch_update_architecture_ftype) (struct gdbarch *gdbarch, const std::vector<cached_reg_t> &regcache);
+extern struct gdbarch * gdbarch_update_architecture (struct gdbarch *gdbarch, const std::vector<cached_reg_t> &regcache);
+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 0edae7f6f0a2..4c9482f8fbf7 100644
--- a/gdb/gdbarch.c
+++ b/gdb/gdbarch.c
@@ -253,6 +253,7 @@ struct gdbarch
   gdbarch_type_align_ftype *type_align = nullptr;
   gdbarch_get_pc_address_flags_ftype *get_pc_address_flags = nullptr;
   gdbarch_read_core_file_mappings_ftype *read_core_file_mappings = nullptr;
+  gdbarch_update_architecture_ftype *update_architecture = nullptr;
 };
 
 /* Create a new ``struct gdbarch'' based on information provided by
@@ -368,6 +369,7 @@ gdbarch_alloc (const struct gdbarch_info *info,
   gdbarch->type_align = default_type_align;
   gdbarch->get_pc_address_flags = default_get_pc_address_flags;
   gdbarch->read_core_file_mappings = default_read_core_file_mappings;
+  gdbarch->update_architecture = default_update_architecture;
   /* gdbarch_alloc() */
 
   return gdbarch;
@@ -605,6 +607,7 @@ 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 (__FILE__, __LINE__,
 		    _("verify_gdbarch: the following are invalid ...%s"),
@@ -1438,6 +1441,9 @@ 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);
 }
@@ -5363,3 +5369,20 @@ 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 std::vector<cached_reg_t> &regcache)
+{
+  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, regcache);
+}
+
+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 70f918a7362c..4e6a33dfb686 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -491,6 +491,8 @@ 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;
@@ -1150,6 +1152,10 @@ struct remote_thread_info : public private_thread_info
      to stop for a watchpoint.  */
   CORE_ADDR watch_data_address = 0;
 
+  /* The architecture corresponding to the expedited registers returned
+     in the last stop reply.  */
+  struct gdbarch *expedited_arch = nullptr;
+
   /* Get the thread's resume state.  */
   enum resume_state get_resume_state () const
   {
@@ -1168,6 +1174,8 @@ 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;
+
+    expedited_arch = nullptr;
   }
 
   /* Get the information this thread's pending vCont-resumption.
@@ -1185,6 +1193,8 @@ struct remote_thread_info : public private_thread_info
   void set_resumed ()
   {
     m_resume_state = resume_state::RESUMED;
+
+    expedited_arch = nullptr;
   }
 
 private:
@@ -8072,6 +8082,21 @@ remote_target::process_stop_reply (struct stop_reply *stop_reply,
       /* Expedited registers.  */
       if (!stop_reply->regcache.empty ())
 	{
+	  /* If GDB already knows about this thread, we can give the
+	     architecture-specific code a chance to update the gdbarch based on
+	     the expedited registers.  */
+	  if (find_thread_ptid (this, ptid) != nullptr)
+	    {
+	      stop_reply->arch = gdbarch_update_architecture (stop_reply->arch,
+							      stop_reply->regcache);
+
+	      /* Save stop_reply->arch so that it can be returned by the
+		 thread_architecture method.  */
+	      remote_thread_info *remote_thr = get_remote_thread_info (this,
+								       ptid);
+	      remote_thr->expedited_arch = stop_reply->arch;
+	    }
+
 	  struct regcache *regcache
 	    = get_thread_arch_regcache (this, ptid, stop_reply->arch);
 
@@ -14448,6 +14473,23 @@ remote_target::thread_info_to_thread_handle (struct thread_info *tp)
   return priv->thread_handle;
 }
 
+struct gdbarch *
+remote_target::thread_architecture (ptid_t ptid)
+{
+  thread_info *thr = find_thread_ptid (this, ptid);
+  remote_thread_info *remote_thr = nullptr;
+
+  if (thr != nullptr)
+    remote_thr = get_remote_thread_info (thr);
+
+  if (remote_thr == nullptr || remote_thr->expedited_arch == nullptr)
+    /* The default thread_architecture implementation is the one from
+       process_stratum_target.  */
+    return process_stratum_target::thread_architecture(ptid);
+
+  return remote_thr->expedited_arch;
+}
+
 bool
 remote_target::can_async_p ()
 {