Aarch64 SVE: Support changing vector lengths in gdbserverusers/ahayward/variable_sve

There are two parts to this patch - gdbserver and GDB.

In gdbserver, there needs to be an equivalent of the thread_architecture
method used in GDB. In regcache, validate the tdesc with the
use target_validate_tdesc target function. If this fails then re-obtain
the target descriptor via general setup.
The aarch64 validation step simply checks the value of the VG register to
see if it matches the current kernel value.

In GDB, we have a similar check when receiving a stop reply. Validate the
tdesc using gdbarch_target_description_changed_p. If this fails re-obtain
the target descriptor via general setup - which is done by setting up an
tdep info structure containing the vector length.
The aarch64 validation step checks the value of VG (which is marked as
an expediated register, so is in the stop reply).

2018-08-03  Alan Hayward  <alan.hayward@arm.com>

gdb/
	* aarch64-tdep.c
	(aarch64_target_description_changed_p): Check vector length.
	(aarch64_target_get_tdep_info): Store vector length.
	* remote.c (remote_target::process_stop_reply): Validate tdesc.
	* target-descriptions.c (target_find_description): Pass through info.
	* target-descriptions.h (target_find_description): Add arg.

gdbserver/
	* linux-aarch64-low.c (aarch64_validate_tdesc): Check vector length.
	* regcache.c (get_thread_regcache): Validate tdesc.

6 files changed, 97 insertions, 9 deletions

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 209bc256f35..6f0f5c69168 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -2971,7 +2971,31 @@ aarch64_target_description_changed_p (struct gdbarch *gdbarch,
 				      ptid_t ptid,
 				      VEC (cached_reg_t) *registers)
 {
-  return false;
+  /* Return true if the VG value in the given VEC of registers list does not
+     match the VG value in the current regcache.  */
+
+  cached_reg_t *reg;
+  bool regcache_has_vg = (gdbarch_num_regs (gdbarch) > AARCH64_SVE_VG_REGNUM);
+
+  for (int ix = 0; VEC_iterate (cached_reg_t, registers, ix, reg); ix++)
+    if (reg->num == AARCH64_SVE_VG_REGNUM)
+      {
+	if (!regcache_has_vg)
+	  {
+	    /* No VG in regcache.  */
+	    return true;
+	  }
+
+	struct regcache *regcache = get_thread_arch_regcache (ptid, gdbarch);
+
+	if (regcache->get_register_status (AARCH64_SVE_VG_REGNUM) != REG_VALID)
+	  return true;
+
+	return !regcache->raw_compare (AARCH64_SVE_VG_REGNUM, reg->data, 0);
+      }
+
+  /* VG is not in the given register list.  */
+  return regcache_has_vg;
 }
 
 /* Implement the "target_get_tdep_info" gdbarch method.  */
@@ -2979,7 +3003,21 @@ aarch64_target_description_changed_p (struct gdbarch *gdbarch,
 static union gdbarch_target_info
 aarch64_target_get_tdep_info (VEC (cached_reg_t) *registers)
 {
-  return {0};
+  gdbarch_target_info info = {0};
+
+  /* Use the current VQ value as the tdep info value.  */
+
+  cached_reg_t *reg;
+
+  for (int ix = 0; VEC_iterate (cached_reg_t, registers, ix, reg); ix++)
+    if (reg->num == AARCH64_SVE_VG_REGNUM)
+      {
+	uint64_t vg = *(uint64_t *) reg->data;
+	info.id = (int *) sve_vq_from_vg (vg);
+	return info;
+      }
+
+  return info;
 }
 
 /* Initialize the current architecture based on INFO.  If possible,
diff --git a/gdb/gdbserver/linux-aarch64-low.c b/gdb/gdbserver/linux-aarch64-low.c
index dcc19ce18c0..6cea4b215ae 100644
--- a/gdb/gdbserver/linux-aarch64-low.c
+++ b/gdb/gdbserver/linux-aarch64-low.c
@@ -3042,7 +3042,26 @@ aarch64_supports_hardware_single_step (void)
 static bool
 aarch64_validate_tdesc (struct thread_info *thread)
 {
-  return true;
+  /* For SVE there is a target descriptor for each VL.  Read the current vector
+     length and check if it matches the size of a variable register in the
+     current target descriptor.  */
+
+  int tid = (ptid_of (thread)).lwp ();
+  long vl = sve_vl_from_vq (aarch64_sve_get_vq (tid));
+  struct regcache *regcache = thread_regcache_data (thread);
+  struct process_info *proc;
+
+  /* Non SVE targets always validate as true.  */
+  if (vl == 0)
+    return true;
+
+  /* If there is a register cache, check the z0 register size.  */
+  if (regcache)
+    return (register_size (regcache->tdesc, AARCH64_SVE_Z0_REGNUM) == vl);
+
+  /* Otherwise, check the z0 register size in the description.  */
+  proc = get_thread_process (thread);
+  return (register_size (proc->tdesc, AARCH64_SVE_Z0_REGNUM) == vl);
 }
 
 struct linux_target_ops the_low_target =
diff --git a/gdb/gdbserver/regcache.c b/gdb/gdbserver/regcache.c
index 0ffec534c36..7b879be9e21 100644
--- a/gdb/gdbserver/regcache.c
+++ b/gdb/gdbserver/regcache.c
@@ -28,6 +28,18 @@ get_thread_regcache (struct thread_info *thread, int fetch)
 {
   struct regcache *regcache;
 
+  /* Check the target descriptor is still valid for the current target.  If
+     not, then clear it and create a new one.  */
+  if (!target_validate_tdesc (thread))
+    {
+      /* Clear regcache.  */
+      free_register_cache (thread_regcache_data (thread));
+      set_thread_regcache_data (thread, NULL);
+      regcache = NULL;
+
+      target_arch_setup ();
+    }
+
   regcache = thread_regcache_data (thread);
 
   /* Threads' regcaches are created lazily, because biarch targets add
diff --git a/gdb/remote.c b/gdb/remote.c
index 69e7d93c87e..a54bd014a49 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -7689,9 +7689,24 @@ remote_target::process_stop_reply (struct stop_reply *stop_reply,
       && status->kind != TARGET_WAITKIND_SIGNALLED
       && status->kind != TARGET_WAITKIND_NO_RESUMED)
     {
+      VEC (cached_reg_t) *stop_regs = stop_reply->regcache;
+
       /* Expedited registers.  */
-      if (stop_reply->regcache)
+      if (stop_regs)
 	{
+	  struct gdbarch *gdbarch = target_gdbarch ();
+
+	  /* Check the target descriptor is still valid for the current target.
+	     If not, then clear it find the correct one.  */
+	  if (gdbarch_target_description_changed_p (gdbarch, ptid, stop_regs))
+	    {
+	      gdbarch_target_info info
+		= gdbarch_target_get_tdep_info (gdbarch, stop_regs);
+	      registers_changed ();
+	      target_clear_description ();
+	      target_find_description (info);
+	    }
+
 	  struct regcache *regcache
 	    = get_thread_arch_regcache (ptid, stop_reply->arch);
 	  cached_reg_t *reg;
diff --git a/gdb/target-descriptions.c b/gdb/target-descriptions.c
index 087de141f7f..a56a38ad97d 100644
--- a/gdb/target-descriptions.c
+++ b/gdb/target-descriptions.c
@@ -491,11 +491,12 @@ target_desc_info_free (struct target_desc_info *tdesc_info)
 
 static char *tdesc_filename_cmd_string;
 
-/* Fetch the current target's description, and switch the current
-   architecture to one which incorporates that description.  */
+/* Fetch the current inferior's description, and switch its current
+   architecture to one which incorporates that description.  If given, use the
+   tdep_info when finding the description.  */
 
 void
-target_find_description (void)
+target_find_description (gdbarch_target_info target_info)
 {
   /* If we've already fetched a description from the target, don't do
      it again.  This allows a target to fetch the description early,
@@ -534,6 +535,8 @@ target_find_description (void)
 
       gdbarch_info_init (&info);
       info.target_desc = current_target_desc;
+      info.target_info = target_info;
+
       if (!gdbarch_update_p (info))
 	warning (_("Architecture rejected target-supplied description"));
       else
diff --git a/gdb/target-descriptions.h b/gdb/target-descriptions.h
index 96290b7d97e..1376b02c425 100644
--- a/gdb/target-descriptions.h
+++ b/gdb/target-descriptions.h
@@ -31,9 +31,10 @@ struct target_desc_info;
 struct inferior;
 
 /* Fetch the current inferior's description, and switch its current
-   architecture to one which incorporates that description.  */
+   architecture to one which incorporates that description.  If given, use the
+   tdep_info when finding the description.  */
 
-void target_find_description (void);
+void target_find_description (gdbarch_target_info target_info = {0});
 
 /* Discard any description fetched from the target for the current
    inferior, and switch the current architecture to one with no target