[PATCH] accel/tcg/cpu-exec: fix precise single-stepping after interrupt

[Luc Michel <lmichel@kalray.eu>]

In some cases, cpu->exit_request can be false after handling the
interrupt, leading to another TB being executed instead of returning
to the main loop.

Fix this by returning true unconditionally when in single-step mode.

Fixes: ba3c35d9c4026361fd380b269dc6def9510b7166

Signed-off-by: Luc Michel <lmichel@kalray.eu>
---
Coming back on this issue I worked on with Richard in 2020. The issue is
that when debugging the guest with GDB, the first instruction of the IRQ
handler is missed by GDB (it's still executed though).

It happened to me again in TCG RR mode (but not in MTTCG). It seems that
cpu->exit_request can be false in RR mode when returning from
cc->tcg_ops->cpu_exec_interrupt, leading to cpu_handle_interrupt
returning false and the next TB being executed, instead of the EXCP_DEBUG
being handled.
---
 accel/tcg/cpu-exec.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index 8b4cd6c59d..74d7f83f34 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -796,13 +796,17 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
                 /*
                  * After processing the interrupt, ensure an EXCP_DEBUG is
                  * raised when single-stepping so that GDB doesn't miss the
                  * next instruction.
                  */
-                cpu->exception_index =
-                    (cpu->singlestep_enabled ? EXCP_DEBUG : -1);
-                *last_tb = NULL;
+                if (unlikely(cpu->singlestep_enabled)) {
+                    cpu->exception_index = EXCP_DEBUG;
+                    return true;
+                } else {
+                    cpu->exception_index = -1;
+                    *last_tb = NULL;
+                }
             }
             /* The target hook may have updated the 'cpu->interrupt_request';
              * reload the 'interrupt_request' value */
             interrupt_request = cpu->interrupt_request;
         }
-- 
2.17.1