Re: [Qemu-devel] [patch 1/2] qemu: mempath: prefault pages manually

[Marcelo Tosatti]

On Wed, Oct 09, 2013 at 10:05:44AM +0200, Paolo Bonzini wrote:
> Il 08/10/2013 23:51, Marcelo Tosatti ha scritto:
> > On Tue, Oct 08, 2013 at 10:03:48AM +0200, Paolo Bonzini wrote:
> >> Il 08/10/2013 02:41, Marcelo Tosatti ha scritto:
> >>> +        /* unblock SIGBUS */
> >>> +        pthread_sigmask(SIG_BLOCK, NULL, &oldset);
> >>> +        sigemptyset(&set);
> >>> +        sigaddset(&set, SIGBUS);
> >>> +        pthread_sigmask(SIG_UNBLOCK, &set, NULL);
> >>
> >> Please instead modify qemu-thread-posix.c to unblock all per-thread
> >> signals (SIGBUS, SIGSEGV, SIGILL, SIGFPE and SIGSYS).  There is no need
> >> to keep those blocked.
> > 
> > main-loop.c handles SIGBUS via signalfd to emulate MCEs (associated
> > commits). Therefore it must be blocked.
> 
> How was that tested?  For BUS_MCEERR_AO it can work, but BUS_MCEERR_AR
> calls force_sig_info which does this:
> 
>         ignored = action->sa.sa_handler == SIG_IGN;
>         blocked = sigismember(&t->blocked, sig);
>         if (blocked || ignored) {
>                 action->sa.sa_handler = SIG_DFL;
>                 if (blocked) {
>                         sigdelset(&t->blocked, sig);
>                         recalc_sigpending_and_wake(t);
>                 }
>         
>         if (action->sa.sa_handler == SIG_DFL)
>                 t->signal->flags &= ~SIGNAL_UNKILLABLE;
> 
> and kills the process (because that's the default action of SIG_DFL).

For vcpu context its not blocked?

> > Note that what this patch does it to maintain the signal handling state
> > (it saves the previous state, modifies state, restores previous state) so 
> > that its unchanged.
> 
> Yes, understood.  I was missing the part about MCE (I knew it used
> SIGBUS, but forgot about signalfd).  So this patch is good, but the
> above point about BUS_MCEERR_AR needs to be checked sooner or later.
> 
> Paolo