[PATCH v2 4/6] gdbserver/linux-aarch64: When thread stops, update its target description

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

This change allows aarch64-linux to support debugging programs where
different threads have different SVE vector lengths.  It requires
gdbserver to support different inferior threads having different target
descriptions.

The arch_update_tdesc method is added to the linux_process_target class to
allow aarch64-linux to probe the inferior's vq register and provide an
updated thread target description reflecting the new vector length.

After this change, all targets except SVE-supporting aarch64-linux will
still use per-process target descriptions.
---
 gdbserver/gdbthread.h          |  2 ++
 gdbserver/linux-aarch64-low.cc | 37 +++++++++++++++++++++++++++++++++-
 gdbserver/linux-low.cc         | 18 +++++++++++++++++
 gdbserver/linux-low.h          |  5 +++++
 gdbserver/regcache.cc          | 11 +++++++---
 gdbserver/tdesc.cc             |  3 +++
 6 files changed, 72 insertions(+), 4 deletions(-)

diff --git a/gdbserver/gdbthread.h b/gdbserver/gdbthread.h
index 8b897e73d33b..47b44d03b8e0 100644
--- a/gdbserver/gdbthread.h
+++ b/gdbserver/gdbthread.h
@@ -80,6 +80,8 @@ struct thread_info
 
   /* Branch trace target information for this thread.  */
   struct btrace_target_info *btrace = nullptr;
+
+  const struct target_desc *tdesc = nullptr;
 };
 
 extern std::list<thread_info *> all_threads;
diff --git a/gdbserver/linux-aarch64-low.cc b/gdbserver/linux-aarch64-low.cc
index cab4fc0a4674..786ce4071279 100644
--- a/gdbserver/linux-aarch64-low.cc
+++ b/gdbserver/linux-aarch64-low.cc
@@ -99,6 +99,9 @@ protected:
 
   void low_arch_setup () override;
 
+  gdb::optional<const struct target_desc *>
+    arch_update_tdesc (const thread_info *thread) override;
+
   bool low_cannot_fetch_register (int regno) override;
 
   bool low_cannot_store_register (int regno) override;
@@ -184,6 +187,8 @@ struct arch_process_info
      same for each thread, it is reasonable for the data to live here.
      */
   struct aarch64_debug_reg_state debug_reg_state;
+
+  bool has_sve;
 };
 
 /* Return true if the size of register 0 is 8 byte.  */
@@ -840,8 +845,16 @@ aarch64_target::low_arch_setup ()
     {
       struct aarch64_features features
 	  = aarch64_get_arch_features (current_thread);
+      const target_desc *tdesc = aarch64_linux_read_description (features);
 
-      current_process ()->tdesc = aarch64_linux_read_description (features);
+      /* Only SVE-enabled inferiors need per-thread target descriptions.  */
+      if (features.vq > 0)
+	{
+	  current_thread->tdesc = tdesc;
+	  current_process ()->priv->arch_private->has_sve = true;
+	}
+
+      current_process ()->tdesc = tdesc;
 
       /* Adjust the register sets we should use for this particular set of
 	 features.  */
@@ -853,6 +866,28 @@ aarch64_target::low_arch_setup ()
   aarch64_linux_get_debug_reg_capacity (lwpid_of (current_thread));
 }
 
+/* Implementation of linux target ops method "arch_update_tdesc".  */
+
+gdb::optional<const struct target_desc *>
+aarch64_target::arch_update_tdesc (const thread_info *thread)
+{
+  /* Only processes using SVE need to update the thread's target description.  */
+  if (!get_thread_process (thread)->priv->arch_private->has_sve)
+    return {};
+
+  const struct aarch64_features features = aarch64_get_arch_features (thread);
+  const struct target_desc *tdesc = aarch64_linux_read_description (features);
+
+  if (tdesc == thread->tdesc)
+    return {};
+
+  /* Adjust the register sets we should use for this particular set of
+     features.  */
+  aarch64_adjust_register_sets(features);
+
+  return tdesc;
+}
+
 /* Implementation of linux target ops method "get_regs_info".  */
 
 const regs_info *
diff --git a/gdbserver/linux-low.cc b/gdbserver/linux-low.cc
index fc496275d6a3..7c510e26f0f5 100644
--- a/gdbserver/linux-low.cc
+++ b/gdbserver/linux-low.cc
@@ -483,6 +483,12 @@ linux_process_target::arch_setup_thread (thread_info *thread)
   low_arch_setup ();
 }
 
+gdb::optional<const struct target_desc *>
+linux_process_target::arch_update_tdesc (const thread_info *thread)
+{
+  return {};
+}
+
 int
 linux_process_target::handle_extended_wait (lwp_info **orig_event_lwp,
 					    int wstat)
@@ -2348,6 +2354,18 @@ linux_process_target::filter_event (int lwpid, int wstat)
 	      return;
 	    }
 	}
+      else
+	{
+	  /* Give the arch code an opportunity to update the thread's target
+	     description.  */
+	  gdb::optional<const struct target_desc *> new_tdesc
+	      = arch_update_tdesc (thread);
+	  if (new_tdesc.has_value ())
+	    {
+	      regcache_release ();
+	      thread->tdesc = *new_tdesc;
+	    }
+	}
     }
 
   if (WIFSTOPPED (wstat) && child->must_set_ptrace_flags)
diff --git a/gdbserver/linux-low.h b/gdbserver/linux-low.h
index 182a540f3bb3..ff14423e9e07 100644
--- a/gdbserver/linux-low.h
+++ b/gdbserver/linux-low.h
@@ -604,6 +604,11 @@ class linux_process_target : public process_stratum_target
   /* Architecture-specific setup for the current thread.  */
   virtual void low_arch_setup () = 0;
 
+  /* Allows arch-specific code to update the thread's target description when
+     the inferior stops.  */
+  virtual gdb::optional<const struct target_desc *>
+    arch_update_tdesc (const thread_info *thread);
+
   /* Return false if we can fetch/store the register, true if we cannot
      fetch/store the register.  */
   virtual bool low_cannot_fetch_register (int regno) = 0;
diff --git a/gdbserver/regcache.cc b/gdbserver/regcache.cc
index 14236069f712..03ee88b3cfd1 100644
--- a/gdbserver/regcache.cc
+++ b/gdbserver/regcache.cc
@@ -39,11 +39,16 @@ get_thread_regcache (struct thread_info *thread, int fetch)
      have.  */
   if (regcache == NULL)
     {
-      struct process_info *proc = get_thread_process (thread);
+      /* First see if there's a thread-specific target description.  */
+      const target_desc *tdesc = thread->tdesc;
 
-      gdb_assert (proc->tdesc != NULL);
+      /* If not, get it from the process instead.  */
+      if (tdesc == nullptr)
+	tdesc = get_thread_process (thread)->tdesc;
 
-      regcache = new_register_cache (proc->tdesc);
+      gdb_assert (tdesc != nullptr);
+
+      regcache = new_register_cache (tdesc);
       set_thread_regcache_data (thread, regcache);
     }
 
diff --git a/gdbserver/tdesc.cc b/gdbserver/tdesc.cc
index 5693cc6626fb..3665ab0540d5 100644
--- a/gdbserver/tdesc.cc
+++ b/gdbserver/tdesc.cc
@@ -129,6 +129,9 @@ current_target_desc (void)
   if (current_thread == NULL)
     return &default_description;
 
+  if (current_thread->tdesc != nullptr)
+    return current_thread->tdesc;
+
   return current_process ()->tdesc;
 }