[RFC PATCH] tcg/cpu-exec: precise single-stepping after an exception

[Luc Michel <luc.michel@greensocs.com>]

When single-stepping with a debugger attached to QEMU, and when an
exception is raised, the debugger misses the first instruction after the
exception:

$ qemu-system-aarch64 -M virt -display none -cpu cortex-a53 -s -S

$ aarch64-linux-gnu-gdb
GNU gdb (GDB) 9.2
[...]
(gdb) tar rem :1234
Remote debugging using :1234
warning: No executable has been specified and target does not support
determining executable automatically.  Try using the "file" command.
0x0000000000000000 in ?? ()
(gdb) # writing nop insns to 0x200 and 0x204
(gdb) set *0x200 = 0xd503201f
(gdb) set *0x204 = 0xd503201f
(gdb) # 0x0 address contains 0 which is an invalid opcode.
(gdb) # The CPU should raise an exception and jump to 0x200
(gdb) si
0x0000000000000204 in ?? ()

With this commit, the same run steps correctly on the first instruction
of the exception vector:

(gdb) si
0x0000000000000200 in ?? ()

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
---

RFC because I'm really not sure this is the good place to do that since
EXCP_DEBUG are usually raised in each target translate.c. It could also
have implications with record/replay I'm not aware of.

---
 accel/tcg/cpu-exec.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index d95c4848a4..e85fab5d40 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -502,10 +502,21 @@ static inline bool cpu_handle_exception(CPUState *cpu, int *ret)
             CPUClass *cc = CPU_GET_CLASS(cpu);
             qemu_mutex_lock_iothread();
             cc->do_interrupt(cpu);
             qemu_mutex_unlock_iothread();
             cpu->exception_index = -1;
+
+            if (unlikely(cpu->singlestep_enabled)) {
+                /*
+                 * After processing the exception, ensure an EXCP_DEBUG is
+                 * raised when single-stepping so that GDB doesn't miss the
+                 * next instruction.
+                 */
+                cpu->exception_index = EXCP_DEBUG;
+                return cpu_handle_exception(cpu, ret);
+            }
+
         } else if (!replay_has_interrupt()) {
             /* give a chance to iothread in replay mode */
             *ret = EXCP_INTERRUPT;
             return true;
         }
-- 
2.27.0