[PATCH] [gdb/aarch64] Handle unknown debug architecture versions more gracefully

[Luis Machado <luis.machado@arm.com>]

In PR 30340 it was reported that a KVM-based AArch64 system's kernel was
returning a debug architecture version of 0 when fetching both the
NT_ARM_HW_WATCH and NT_ARM_HW_BREAK register sets.

Even though the debug architecture version being reported is invalid, gdb can
still make things work by ignoring that information and relying on the counts
of hardware watchpoints and hardware breakpoints.

This patch makes gdb handle this situation a bit more gracefully by causing gdb
to warn when it sees an invalid/unknown debug architecture version, but still
allowing gdb to detect the number of hardware watchpoints and hardware
breakpoints available.

PR tdep/30340
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30340
---
 gdb/nat/aarch64-linux-hw-point.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/gdb/nat/aarch64-linux-hw-point.c b/gdb/nat/aarch64-linux-hw-point.c
index ccb47cd5aa2..b014d387e1e 100644
--- a/gdb/nat/aarch64-linux-hw-point.c
+++ b/gdb/nat/aarch64-linux-hw-point.c
@@ -250,10 +250,21 @@ aarch64_linux_get_debug_reg_capacity (int tid)
   iov.iov_base = &dreg_state;
   iov.iov_len = sizeof (dreg_state);
 
+  /* It has been reported that under KVM the debug architecture version can
+     be reported as 0, which is invalid.  But this doesn't mean gdb can't use
+     hardware debugging resources.  Instead of bailing out, carry on fetching
+     the hardware breakpoints/watchpoints count so we can potentially get back
+     on track.  */
+
   /* Get hardware watchpoint register info.  */
-  if (ptrace (PTRACE_GETREGSET, tid, NT_ARM_HW_WATCH, &iov) == 0
-      && compatible_debug_arch (AARCH64_DEBUG_ARCH (dreg_state.dbg_info)))
+  if (ptrace (PTRACE_GETREGSET, tid, NT_ARM_HW_WATCH, &iov) == 0)
     {
+      if (!compatible_debug_arch (AARCH64_DEBUG_ARCH (dreg_state.dbg_info)))
+	warning (_("Unknown/Invalid debug architecture version %d.\n"
+		   "Attempting to fetch the number of hardware watchpoints "
+		   "available."),
+		   AARCH64_DEBUG_ARCH (dreg_state.dbg_info));
+
       aarch64_num_wp_regs = AARCH64_DEBUG_NUM_SLOTS (dreg_state.dbg_info);
       if (aarch64_num_wp_regs > AARCH64_HWP_MAX_NUM)
 	{
@@ -271,9 +282,14 @@ aarch64_linux_get_debug_reg_capacity (int tid)
     }
 
   /* Get hardware breakpoint register info.  */
-  if (ptrace (PTRACE_GETREGSET, tid, NT_ARM_HW_BREAK, &iov) == 0
-      && compatible_debug_arch (AARCH64_DEBUG_ARCH (dreg_state.dbg_info)))
+  if (ptrace (PTRACE_GETREGSET, tid, NT_ARM_HW_BREAK, &iov) == 0)
     {
+      if (!compatible_debug_arch (AARCH64_DEBUG_ARCH (dreg_state.dbg_info)))
+	warning (_("Unknown/Invalid debug architecture version %d.\n"
+		   "Attempting to fetch the number of hardware breakpoints "
+		   "available."),
+		   AARCH64_DEBUG_ARCH (dreg_state.dbg_info));
+
       aarch64_num_bp_regs = AARCH64_DEBUG_NUM_SLOTS (dreg_state.dbg_info);
       if (aarch64_num_bp_regs > AARCH64_HBP_MAX_NUM)
 	{
-- 
2.25.1