[Qemu-devel] Re: [PATCH 14/22] kvm: Fix race between timer signals and vcpu entry under !IOTHREAD

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-02-01 15:10, Marcelo Tosatti wrote:
> On Tue, Feb 01, 2011 at 02:58:02PM +0100, Jan Kiszka wrote:
>> On 2011-02-01 14:48, Marcelo Tosatti wrote:
>>> On Tue, Feb 01, 2011 at 02:32:38PM +0100, Jan Kiszka wrote:
>>>> On 2011-02-01 13:47, Marcelo Tosatti wrote:
>>>>> On Thu, Jan 27, 2011 at 02:09:58PM +0100, Jan Kiszka wrote:
>>>>>> Found by Stefan Hajnoczi: There is a race in kvm_cpu_exec between
>>>>>> checking for exit_request on vcpu entry and timer signals arriving
>>>>>> before KVM starts to catch them. Plug it by blocking both timer related
>>>>>> signals also on !CONFIG_IOTHREAD and process those via signalfd.
>>>>>>
>>>>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>>>>> CC: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
>>>>>> ---
>>>>>>  cpus.c |   18 ++++++++++++++++++
>>>>>>  1 files changed, 18 insertions(+), 0 deletions(-)
>>>>>>
>>>>>> diff --git a/cpus.c b/cpus.c
>>>>>> index fc3f222..29b1070 100644
>>>>>> --- a/cpus.c
>>>>>> +++ b/cpus.c
>>>>>> @@ -254,6 +254,10 @@ static void qemu_kvm_init_cpu_signals(CPUState *env)
>>>>>>      pthread_sigmask(SIG_BLOCK, NULL, &set);
>>>>>>      sigdelset(&set, SIG_IPI);
>>>>>>      sigdelset(&set, SIGBUS);
>>>>>> +#ifndef CONFIG_IOTHREAD
>>>>>> +    sigdelset(&set, SIGIO);
>>>>>> +    sigdelset(&set, SIGALRM);
>>>>>> +#endif
>>>>>
>>>>> I'd prefer separate qemu_kvm_init_cpu_signals in the !IOTHREAD
>>>>> section.
>>>>
>>>> You mean to duplicate qemu_kvm_init_cpu_signals for both configurations?
>>>
>>> Yes, so to avoid #ifdefs spread.
>>
>> Would exchange some #ifdefs against ifndef _WIN32. Haven't measured the
>> delta though.
>>
>>>
>>>>>> +
>>>>>> +#ifndef CONFIG_IOTHREAD
>>>>>> +    if (sigismember(&chkset, SIGIO) || sigismember(&chkset, SIGALRM)) {
>>>>>> +        qemu_notify_event();
>>>>>> +    }
>>>>>> +#endif
>>>>>
>>>>> Why is this necessary?
>>>>>
>>>>> You should break out of cpu_exec_all if there's a pending alarm (see
>>>>> qemu_alarm_pending()).
>>>>
>>>> qemu_alarm_pending() is not true until the signal is actually taken. The
>>>> alarm handler sets the required flags.
>>>
>>> Right. What i mean is you need to execute the signal handler inside
>>> cpu_exec_all loop (so that alarm pending is set).
>>>
>>> So, if there is a SIGALRM pending, qemu_run_timers has highest
>>> priority, not vcpu execution.
>>
>> We leave the vcpu loop (thanks to notify_event), process the signal in
>> the event loop and run the timer handler. This pattern is IMO less
>> invasive to the existing code, specifically as it is about to die
>> long-term anyway.
> 
> You'll probably see poor timer behaviour on smp guests without iothread
> enabled.
> 

Still checking, but that would mean the notification mechanism is broken
anyway: If IO events do not force us to process them quickly, we already
suffer from latencies in SMP mode.

Jan

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