linux-kernel - Re: [PATCH 4/5] KVM: x86/mmu: Don't install TDP MMU SPTE if SP has unexpected level

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Date:   Tue, 13 Dec 2022 18:15:56 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Robert Hoo <robert.hu@...ux.intel.com>,
        Greg Thelen <gthelen@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH 4/5] KVM: x86/mmu: Don't install TDP MMU SPTE if SP has
 unexpected level

On Tue, Dec 13, 2022, David Matlack wrote:
> On Mon, Dec 12, 2022 at 7:30 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > Don't install a leaf TDP MMU SPTE if the parent page's level doesn't
> > match the target level of the fault, and instead have the vCPU retry the
> > faulting instruction after warning.  Continuing on is completely
> > unnecessary as the absolute worst case scenario of retrying is DoSing
> > the vCPU, whereas continuing on all but guarantees bigger explosions, e.g.
> 
> Would it make sense to kill the VM instead via KVM_BUG()?

No, because if bug that hits this escapes to a release, odds are quite high that
retrying will succeed.  E.g. the fix earlier in this series is for a rare corner
case that I was able to hit consistently only by hacking KVM to effectively
synchronize the page fault and zap.  Other than an extra page fault, no harm has
been done to the guest, e.g. there's no need to kill the VM to protect it from
data corruption.