Date:   Tue, 7 Mar 2023 16:39:24 -0800
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Subject: Re: [PATCH] KVM: SVM: Disable TDP MMU when running on Hyper-V
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Tianyu Lan <ltykernel@gmail.com>,
        Michael Kelley <mikelley@microsoft.com>
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On Wed, Mar 08, 2023, Paolo Bonzini wrote:
> On Tue, Mar 7, 2023 at 6:36=E2=80=AFPM Sean Christopherson <seanjc@google=
.com> wrote:
> > Thinking about this more, I would rather revert commit 1e0c7d40758b ("K=
VM: SVM:
> > hyper-v: Remote TLB flush for SVM") or fix the thing properly straitawa=
y.  KVM
> > doesn't magically handle the flushes correctly for the shadow/legacy MM=
U, KVM just
> > happens to get lucky and not run afoul of the underlying bugs.
>=20
> I don't think it's about luck---the legacy MMU's zapping/invalidation
> seems to invoke the flush hypercall correctly:

...for the paths that Jeremi has exercised, and for which a stale TLB entry=
 is
fatal to L2.  E.g. kvm_unmap_gfn_range() does not have a range-based TLB fl=
ush
in its path and fully relies on the buggy kvm_flush_remote_tlbs().

In other words, KVM is getting lucky :-)

> Jeremi, did you ever track the call stack where
> hyperv_nested_flush_guest_mapping is triggered?

I don't think it matters.  As above, it only takes one path where KVM is fu=
lly
relying on kvm_flush_remote_tlbs() for the whole thing to fall apart.