linux-kernel - Re: [PATCH MANUALSEL 5.15 1/4] KVM: x86: do not report a vCPU as preempted outside instruction boundaries

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Message-ID: <CAG48ez3nxe32Hv3dXO27_rK3qrSGZUW8Pp1sxLDxwKWkL1BaoQ@mail.gmail.com>
Date:   Wed, 15 Jun 2022 16:58:02 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Sasha Levin <sashal@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.15 1/4] KVM: x86: do not report a vCPU as
 preempted outside instruction boundaries

On Wed, Jun 15, 2022 at 4:53 PM Jann Horn <jannh@...gle.com> wrote:
>
> On Tue, Jun 14, 2022 at 4:11 AM Sasha Levin <sashal@...nel.org> wrote:
> >
> > From: Paolo Bonzini <pbonzini@...hat.com>
> >
> > [ Upstream commit 6cd88243c7e03845a450795e134b488fc2afb736 ]
> >
> > If a vCPU is outside guest mode and is scheduled out, it might be in the
> > process of making a memory access.  A problem occurs if another vCPU uses
> > the PV TLB flush feature during the period when the vCPU is scheduled
> > out, and a virtual address has already been translated but has not yet
> > been accessed, because this is equivalent to using a stale TLB entry.
> >
> > To avoid this, only report a vCPU as preempted if sure that the guest
> > is at an instruction boundary.  A rescheduling request will be delivered
> > to the host physical CPU as an external interrupt, so for simplicity
> > consider any vmexit *not* instruction boundary except for external
> > interrupts.
> >
> > It would in principle be okay to report the vCPU as preempted also
> > if it is sleeping in kvm_vcpu_block(): a TLB flush IPI will incur the
> > vmentry/vmexit overhead unnecessarily, and optimistic spinning is
> > also unlikely to succeed.  However, leave it for later because right
> > now kvm_vcpu_check_block() is doing memory accesses.  Even
> > though the TLB flush issue only applies to virtual memory address,
> > it's very much preferrable to be conservative.
> >
> > Reported-by: Jann Horn <jannh@...gle.com>
> > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> > Signed-off-by: Sasha Levin <sashal@...nel.org>
>
> This feature was introduced in commit f38a7b75267f1f (first in 4.16).
> I think the fix has to be applied all the way back to there (so
> additionally to what you already did, it'd have to be added to 4.19,
> 5.4 and 5.10)?
>
> But it doesn't seem to apply cleanly to those older branches. Paolo,
> are you going to send stable backports of this?

Also, I think the same thing applies for "KVM: x86: do not set
st->preempted when going back to user space"?