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From:   Jim Mattson <jmattson@google.com>
Date:   Wed, 31 Jul 2019 13:27:53 -0700
Message-ID: <CALMp9eQLCEzfdNzdhPtCf3bD-5c6HrSvJqP7idyoo4Gf3i5O1w@mail.gmail.com>
Subject: Re: [PATCH RFC 3/5] x86: KVM: svm: clear interrupt shadow on all
 paths in skip_emulated_instruction()
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Joerg Roedel <joro@8bytes.org>
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On Wed, Jul 31, 2019 at 9:37 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 31/07/19 15:50, Vitaly Kuznetsov wrote:
> > Jim Mattson <jmattson@google.com> writes:
> >
> >> On Thu, Jun 20, 2019 at 4:02 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> >>>
> >>> Regardless of the way how we skip instruction, interrupt shadow needs to be
> >>> cleared.
> >>
> >> This change is definitely an improvement, but the existing code seems
> >> to assume that we never call skip_emulated_instruction on a
> >> POP-SS/MOV-to-SS/STI. Is that enforced anywhere?
> >
> > (before I send v1 of the series) I looked at the current code and I
> > don't think it is enforced, however, VMX version does the same and
> > honestly I can't think of a situation when we would be doing 'skip' for
> > such an instruction.... and there's nothing we can easily enforce from
> > skip_emulated_instruction() as we have no idea what the instruction
> > is...

Can't we still coerce kvm into emulating any instruction by leveraging
a stale ITLB entry? The 'emulator' kvm-unit-test did this before the
KVM forced emulation prefix was introduced, but I haven't checked to
see if the original (admittedly fragile) approach still works. Also,
for POP-SS, you could always force emulation by mapping the %rsp
address beyond guest physical memory. The hypervisor would then have
to emulate the instruction to provide bus-error semantics.

> I agree, I think a comment is worthwhile but we can live with the
> limitation.

I think we can live with the limitation, but I'd really prefer to see
a KVM exit with KVM_INTERNAL_ERROR_EMULATION for an instruction that
kvm doesn't emulate properly. That seems better than just a comment
that the virtual CPU doesn't behave as architected. (I realize that I
am probably in the minority here.)