linux-kernel - Re: [PATCH RFC 3/5] x86: KVM: svm: clear interrupt shadow on all paths in skip_emulated

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Date:   Wed, 31 Jul 2019 17:17:18 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH RFC 3/5] x86: KVM: svm: clear interrupt shadow on all
 paths in skip_emulated_instruction()

On Wed, Jul 31, 2019 at 5:13 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 01/08/19 01:56, Sean Christopherson wrote:
> > On Wed, Jul 31, 2019 at 04:45:21PM -0700, Jim Mattson wrote:
> >> On Wed, Jul 31, 2019 at 4:37 PM Sean Christopherson
> >> <sean.j.christopherson@...el.com> wrote:
> >>
> >>> At a glance, the full emulator models behavior correctly, e.g. see
> >>> toggle_interruptibility() and setters of ctxt->interruptibility.
> >>>
> >>> I'm pretty sure that leaves the EPT misconfig MMIO and APIC access EOI
> >>> fast paths as the only (VMX) path that would incorrectly handle a
> >>> MOV/POP SS.  Reading the guest's instruction stream to detect MOV/POP SS
> >>> would defeat the whole "fast path" thing, not to mention both paths aren't
> >>> exactly architecturally compliant in the first place.
> >>
> >> The proposed patch clears the interrupt shadow in the VMCB on all
> >> paths through svm's skip_emulated_instruction. If this happens at the
> >> tail end of emulation, it doesn't matter if the full emulator does the
> >> right thing.
> >
> > Unless I'm missing something, skip_emulated_instruction() isn't called in
> > the emulation case, x86_emulate_instruction() updates %rip directly, e.g.:
>
> Indeed.  skip_emulated_instruction() is only used when the vmexit code
> takes care of emulation directly.

Mea culpa. I had incorrectly assumed that "skip_emulated_instruction"
was used when an instruction was emulated. I retract my objection.
Having now been twice bitten by misleading function names, I'll be
more careful in the future.