Re: [PATCH v2 0/7] kvm: fix latent guest entry/exit bugs

[Christian Borntraeger]

Am 20.01.22 um 13:03 schrieb Mark Rutland:
> On Thu, Jan 20, 2022 at 12:28:09PM +0100, Paolo Bonzini wrote:
> > On 1/19/22 20:22, Mark Rutland wrote:
> > > I wonder, is the s390 guest entry/exit*preemptible*  ?
> > >
> > > If a timer IRQ can preempt in the middle of the EQS, we wouldn't balance
> > > things before a ctx-switch to the idle thread, which would then be able
> > > to hit this.
> > >
> > > I'll need to go audit the other architectures for similar.
> >
> > They don't enable interrupts in the entry/exit path so they should be okay.
>
> True.
>
> So it sounds like for s390 adding an explicit preempt_{disable,enable}() is the
> right thing to do. I'll add that and explanatory commentary.

That would not be trivial I guess. We do allow (and need) page faults on sie for guest
demand paging and

this piece of arch/s390/mm/fault.c

       case GMAP_FAULT:
                if (faulthandler_disabled() || !mm)
                        goto out;
                break;
        }

would no longer work since faulthandler_disabled checks for the preempt count.