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

[Marcelo Tosatti <mtosatti@redhat.com>]

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.