linux-kernel - Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by VM instructions

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Date:   Tue, 12 Jan 2021 10:58:29 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Wei Huang <wei.huang2@....com>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Bandan Das <bsd@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by
 VM instructions

On Tue, Jan 12, 2021 at 9:59 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, Jan 12, 2021, Sean Christopherson wrote:
> > On Tue, Jan 12, 2021, Wei Huang wrote:
> > > From: Bandan Das <bsd@...hat.com>
> > >
> > > While running VM related instructions (VMRUN/VMSAVE/VMLOAD), some AMD
> > > CPUs check EAX against reserved memory regions (e.g. SMM memory on host)
> > > before checking VMCB's instruction intercept.
> >
> > It would be very helpful to list exactly which CPUs are/aren't affected, even if
> > that just means stating something like "all CPUs before XYZ".  Given patch 2/2,
> > I assume it's all CPUs without the new CPUID flag?
>
> Ah, despite calling this an 'errata', the bad behavior is explicitly documented
> in the APM, i.e. it's an architecture bug, not a silicon bug.
>
> Can you reword the changelog to make it clear that the premature #GP is the
> correct architectural behavior for CPUs without the new CPUID flag?

Andrew Cooper points out that there may be a nicer workaround.  Make
sure that the SMRAM and HT region (FFFD00000000 - FFFFFFFFFFFF) are
marked as reserved in the guest, too.

--Andy