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

[Christian Borntraeger]

Am 21.01.22 um 15:30 schrieb Mark Rutland:
> On Fri, Jan 21, 2022 at 03:17:01PM +0100, Christian Borntraeger wrote:
> > Am 21.01.22 um 10:53 schrieb Christian Borntraeger:
> > > Am 20.01.22 um 16:14 schrieb 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.
> > >
> > >
> > > Something like this
> > >
> > >
> > > diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> > > index d30f5986fa85..1c7d45346e12 100644
> > > --- a/arch/s390/mm/fault.c
> > > +++ b/arch/s390/mm/fault.c
> > > @@ -385,10 +385,18 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
> > >                           return 0;
> > >                   goto out;
> > >           case USER_FAULT:
> > > -       case GMAP_FAULT:
> > >                   if (faulthandler_disabled() || !mm)
> > >                           goto out;
> > >                   break;
> > > +               /*
> > > +                * We know that we interrupted SIE and we are not in a IRQ.
> > > +                * preemption might be disabled thus checking for in_atomic
> > > +                * would result in failures
> > > +                */
> > > +       case GMAP_FAULT:
> > > +               if (pagefault_disabled() || !mm)
> > > +                       goto out;
> > > +               break;
> > >           }
> > >
> > >           perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
> > >
> > > seems to work with preemption disabled around sie. Not sure yet if this is correct.
> >
> >
> > No it does not work. scheduling while preemption is disabled.
>
> Hmm... which exceptions do we expect to take from a guest?
>
> I wonder if we can handle those more like other architectures by getting those
> to immediately return from the sie64a() call with some status code that we can
> handle outside of the guest_state_{enter,exit}_irqoff() critical section.

We take all kind of page faults (invalid->valid, ro->rw) on the sie instruction and
run that in the context of the pgm_check handler just like for userspace. the pgm_check
handler does a sie_exit (similar to the interrupt handlers) by setting the return IA.
 
> On arm64 in VHE mode, we swap our exception vectors when entering/exiting the
> guest to allow us to do that; I wonder if we could do similar here?