On Wed, Oct 28, 2009 at 11:42:56AM +0200, Avi Kivity wrote:
> On 10/27/2009 05:33 PM, Marcelo Tosatti wrote:
>> Michael reported a qemu-kvm SIGSEGV at shutdown:
>>
>> Program received signal SIGSEGV, Segmentation fault.
>> [Switching to Thread 0x411d0940 (LWP 14446)]
>> 0x000000000040afb4 in qemu_mod_timer (ts=0x19f0fd0,
>> expire_time=62275467335)
>> at /home/mst/scm/qemu-kvm/vl.c:1009
>> 1009 if ((alarm_timer->flags& ALARM_FLAG_EXPIRED) == 0)
>> {
>> (gdb) l
>> 1004 ts->next = *pt;
>> 1005 *pt = ts;
>> 1006
>> 1007 /* Rearm if necessary */
>> 1008 if (pt ==&active_timers[ts->clock->type]) {
>> 1009 if ((alarm_timer->flags& ALARM_FLAG_EXPIRED) == 0)
>> {
>> 1010 qemu_rearm_alarm_timer(alarm_timer);
>> 1011 }
>> 1012 /* Interrupt execution to force deadline
>> recalculation. */
>> 1013 if (use_icount)
>> (gdb) p alarm_timer
>> $1 = (struct qemu_alarm_timer *) 0x0
>>
>> Problem is kvm_main_loop_wait(env, 0) can process the stop request
>> (signalling iothread that vcpu is stopped, so its OK to exit) and
>> continue to kvm_cpu_exec.
>>
>> Reorder kvm_main_loop_wait and kvm_cpu_exec, as suggested by Gleb.
>>
>> Signed-off-by: Marcelo Tosatti<mtosatti@xxxxxxxxxx>
>> Reported-by: "Michael S. Tsirkin"<mst@xxxxxxxxxx>
>>
>> diff --git a/qemu-kvm.c b/qemu-kvm.c
>> index 4c13628..809fd65 100644
>> --- a/qemu-kvm.c
>> +++ b/qemu-kvm.c
>> @@ -1867,8 +1867,8 @@ static int kvm_main_loop_cpu(CPUState *env)
>> run_cpu = !env->halted;
>> }
>> if (run_cpu) {
>> - kvm_main_loop_wait(env, 0);
>> kvm_cpu_exec(env);
>> + kvm_main_loop_wait(env, 0);
>> } else {
>> kvm_main_loop_wait(env, 1000);
>> }
>>
>
> This will miss an event at the very beginning of the loop (powerdown
> requested before we had a chance to spin up?). I think it should be
> fine since there will be a pending signal which will break us out of
> kvm_cpu_exec() before it has a chance to do anything (i.e. spin in the
> guest).
>
> So, I think it's fine, but please double check the above.
That can happen but only with the unpatched version, where
kvm_main_loop_wait eats the signal, sets env->stopped = 1,
and proceeds to kvm_cpu_exec.
With the patch, this is not the case.
As before, the signal sender will send an IPI with smp_send_reschedule
(and vcpu_enter_guest checks sigpending after disabling interrupts).
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