qemu-devel.nongnu.org archive mirror
 help / color / mirror / Atom feed
* [PATCH] accel/tcg/cpu-exec: fix precise single-stepping after interrupt
@ 2022-02-14 13:26 Luc Michel
  2022-02-23 10:51 ` Luc Michel
  2022-02-25  0:23 ` Richard Henderson
  0 siblings, 2 replies; 6+ messages in thread
From: Luc Michel @ 2022-02-14 13:26 UTC (permalink / raw)
  To: qemu-devel
  Cc: Luc Michel, Richard Henderson, Peter Maydell,
	Philippe Mathieu-Daudé, Paolo Bonzini

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



^ permalink raw reply related	[flat|nested] 6+ messages in thread
* [PATCH 0/7] semihosting: proper QEMU exit on semihosted exit syscall
@ 2022-06-20 14:24 Luc Michel
  2022-06-20 14:24 ` [PATCH] accel/tcg/cpu-exec: fix precise single-stepping after interrupt Luc Michel
  0 siblings, 1 reply; 6+ messages in thread
From: Luc Michel @ 2022-06-20 14:24 UTC (permalink / raw)
  To: qemu-devel
  Cc: Luc Michel, Richard Henderson, Peter Maydell,
	Philippe Mathieu-Daudé, Paolo Bonzini, Alex Bennée,
	Eric Blake, Markus Armbruster, Laurent Vivier, Aurelien Jarno,
	Jiaxun Yang, Aleksandar Rikalo, Chris Wulff, Marek Vasut,
	Max Filippov

Hi,

This series implements a clean way for semihosted exit syscalls to
terminate QEMU with a given return code.

Until now, exit syscalls implementations consisted in calling exit()
with the wanted return code. The problem with this approach is that
other CPUs are not properly stopped, leading to possible crashes in
MTTCG mode, especially when at_exit callbacks have been registered. This
can be the case e.g., when plugins are in use. Plugins can register
at_exit callbacks. Those will be called on the CPU thread the exit
syscall is comming from, while other CPUs can continue to run and thus
call other plugin callbacks.

The semihosting_exit_request function provides a mean to cleanly
terminate QEMU. It introduces an new exit reason
(SHUTDOWN_CAUSE_GUEST_SEMI_EXIT) used in this case. The CPU is stopped
and returns to the main CPU loop so that no more instruction get
executed (the semihosting_exit_request is declared G_NORETURN).

All targets are converted to use this new function.

Thanks,
Luc

Luc Michel (7):
  softmmu: add qemu_[set|get]_exit_status functions
  semihosting: add the semihosting_exit_request function
  semihosting/arm-compat-semi: use semihosting_exit_request
  target/m68k: use semihosting_exit_request on semihosted exit syscall
  target/mips: use semihosting_exit_request on semihosted exit syscall
  target/nios2: use semihosting_exit_request on semihosted exit syscall
  target/xtensa: use semihosting_exit_request on semihosted exit syscall

 qapi/run-state.json                |  4 +++-
 include/semihosting/semihost.h     |  4 ++++
 include/sysemu/sysemu.h            |  2 ++
 semihosting/arm-compat-semi.c      |  3 +--
 semihosting/config.c               | 17 +++++++++++++++++
 softmmu/main.c                     |  2 +-
 softmmu/runstate.c                 | 11 +++++++++++
 target/m68k/m68k-semi.c            |  4 ++--
 target/mips/tcg/sysemu/mips-semi.c |  2 +-
 target/nios2/nios2-semi.c          |  4 ++--
 target/xtensa/xtensa-semi.c        |  2 +-
 11 files changed, 45 insertions(+), 10 deletions(-)

-- 
2.17.1



^ permalink raw reply	[flat|nested] 6+ messages in thread
end of thread, other threads:[~2022-06-20 14:42 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2022-02-14 13:26 [PATCH] accel/tcg/cpu-exec: fix precise single-stepping after interrupt Luc Michel
2022-02-23 10:51 ` Luc Michel
2022-02-25  0:23 ` Richard Henderson
2022-02-25  0:52   ` Richard Henderson
  -- strict thread matches above, loose matches on Subject: below --
2022-06-20 14:24 [PATCH 0/7] semihosting: proper QEMU exit on semihosted exit syscall Luc Michel
2022-06-20 14:24 ` [PATCH] accel/tcg/cpu-exec: fix precise single-stepping after interrupt Luc Michel
2022-06-20 14:27   ` Luc Michel

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).