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

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Date:   Fri, 21 Jan 2022 15:29:50 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Christian Borntraeger <borntraeger@...ux.ibm.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        aleksandar.qemu.devel@...il.com, alexandru.elisei@....com,
        anup.patel@....com, aou@...s.berkeley.edu, atish.patra@....com,
        bp@...en8.de, catalin.marinas@....com, chenhuacai@...nel.org,
        dave.hansen@...ux.intel.com, frankja@...ux.ibm.com,
        frederic@...nel.org, gor@...ux.ibm.com, hca@...ux.ibm.com,
        james.morse@....com, jmattson@...gle.com, joro@...tes.org,
        luto@...nel.org, maz@...nel.org, mingo@...hat.com,
        nsaenzju@...hat.com, palmer@...belt.com, paulmck@...nel.org,
        paul.walmsley@...ive.com, peterz@...radead.org, seanjc@...gle.com,
        suzuki.poulose@....com, svens@...ux.ibm.com, tglx@...utronix.de,
        tsbogend@...ha.franken.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>
Subject: Re: [PATCH v2 0/7] kvm: fix latent guest entry/exit bugs

On Fri, Jan 21, 2022 at 03:42:48PM +0100, Christian Borntraeger wrote:
> 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.

Do we only expect to tak faults from a guest (which IIUC at the GMAP_FAULT
cases in the bit above), or are there other esceptions we expect to take from
the middle of a SIE?

> the pgm_check handler does a sie_exit (similar to the interrupt handlers) by
> setting the return IA.

Sure, but that sie_exit happens *after* the handler runs, where as I'm asking
whether we can structure this like all the other architectures and turn all
exceptions into returns from sie64a() that we can handle after having called
guest_state_exit_irqoff().

> > 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?

Does this idea sound at all plausible?

Thanks,
Mark.