linux-kernel - Re: [PATCH v4 2/2] KVM: SVM: Add support for Virtual SPEC

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Message-ID: <CALMp9eRnC1RgRwj64TJcXdhhL6g835N_-E8FbeHVre6aX=18-A@mail.gmail.com>
Date:   Mon, 13 Jun 2022 12:23:40 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Babu Moger <babu.moger@....com>, pbonzini@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        fenghua.yu@...el.com, tony.luck@...el.com, wanpengli@...cent.com,
        kvm@...r.kernel.org, peterz@...radead.org, seanjc@...gle.com,
        joro@...tes.org, x86@...nel.org, kyung.min.park@...el.com,
        linux-kernel@...r.kernel.org, krish.sadhukhan@...cle.com,
        hpa@...or.com, mgross@...ux.intel.com, vkuznets@...hat.com,
        kim.phillips@....com, wei.huang2@....com
Subject: Re: [PATCH v4 2/2] KVM: SVM: Add support for Virtual SPEC_CTRL

On Mon, Jun 13, 2022 at 8:10 AM Tom Lendacky <thomas.lendacky@....com> wrote:
>
> On 6/3/22 22:11, Jim Mattson wrote:
> > On Thu, Jan 28, 2021 at 4:43 PM Babu Moger <babu.moger@....com> wrote:
> >
> >> This support also fixes an issue where a guest may sometimes see an
> >> inconsistent value for the SPEC_CTRL MSR on processors that support
> >> this feature. With the current SPEC_CTRL support, the first write to
> >> SPEC_CTRL is intercepted and the virtualized version of the SPEC_CTRL
> >> MSR is not updated. When the guest reads back the SPEC_CTRL MSR, it
> >> will be 0x0, instead of the actual expected value. There isn’t a
> >> security concern here, because the host SPEC_CTRL value is or’ed with
> >> the Guest SPEC_CTRL value to generate the effective SPEC_CTRL value.
> >> KVM writes with the guest's virtualized SPEC_CTRL value to SPEC_CTRL
> >> MSR just before the VMRUN, so it will always have the actual value
> >> even though it doesn’t appear that way in the guest. The guest will
> >> only see the proper value for the SPEC_CTRL register if the guest was
> >> to write to the SPEC_CTRL register again. With Virtual SPEC_CTRL
> >> support, the save area spec_ctrl is properly saved and restored.
> >> So, the guest will always see the proper value when it is read back.
> >
> > Note that there are actually two significant problems with the way the
> > new feature interacts with the KVM code before this patch:
> > 1) All bits set by the first non-zero write become sticky until the
> > vCPU is reset (because svm->spec_ctrl is never modified after the
> > first non-zero write).
>
> When X86_FEATURE_V_SPEC_CTRL is set, then svm->spec_ctrl isn't used.

Post-patch, yes. I'm talking about how this new hardware feature broke
versions of KVM *before* this patch was submitted.

> > 2) The current guest IA32_SPEC_CTRL value isn't actually known to the
> > hypervisor.
>
> The hypervisor can read the value as long as it is not an SEV-ES or
> SEV-SNP guest.

Yes, it can, but KVM prior to this patch did not. Again, I'm talking
about how this new hardware feature broke *existing* versions of KVM.

> > It thinks that there are no writes to the MSR after the
> > first non-zero write, so that sticky value will be returned to
> > KVM_GET_MSRS. This breaks live migration.
>
> KVM_GET_MSRS should go through the normal read MSR path, which will read
> the guest MSR value from either svm->vmcb->save.spec_ctrl if
> X86_FEATURE_V_SPEC_CTRL is set or from svm->spec_ctrl, otherwise. And the
> write MSR path will do similar.

You really are gaslighting me, aren't you?

> I'm probably missing something here, because I'm not good with the whole
> live migration thing as it relates to host and guest features.

AMD added a new VMCB field that existing hypervisors knew nothing
about. This VMCB field contains the current value of the guest's
IA32_SPEC_CTRL. Since the hypervisor doesn't know that this field even
exists, it cannot migrate it.