linux-kernel - Re: [PATCH v2 11/11] KVM: x86: emulator/smm: preserve interrupt shadow in SMRAM

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Date:   Wed, 6 Jul 2022 11:13:36 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
        x86@...nel.org, Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        Borislav Petkov <bp@...en8.de>, Joerg Roedel <joro@...tes.org>,
        Ingo Molnar <mingo@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [PATCH v2 11/11] KVM: x86: emulator/smm: preserve interrupt
 shadow in SMRAM

On Tue, Jul 5, 2022 at 6:38 AM Maxim Levitsky <mlevitsk@...hat.com> wrote:

> Most of the SMI save state area is reserved, and the handler has no way of knowing
> what CPU stored there, it can only access the fields that are reserved in the spec.
>
> Yes, if the SMI handler really insists it can see that the saved RIP points to an
> instruction that follows the STI, but does that really matter? It is allowed by the
> spec explicitly anyway.

I was just pointing out that the difference between blocking SMI and
not blocking SMI is, in fact, observable.

> Plus our SMI layout (at least for 32 bit) doesn't confirm to the X86 spec anyway,
> we as I found out flat out write over the fields that have other meaning in the X86 spec.

Shouldn't we fix that?

> Also I proposed to preserve the int shadow in internal kvm state and migrate
> it in upper 4 bits of the 'shadow' field of struct kvm_vcpu_events.
> Both Paolo and Sean proposed to store the int shadow in the SMRAM instead,
> and you didn't object to this, and now after I refactored and implemented
> the whole thing you suddently do.

I did not see the prior conversations. I rarely get an opportunity to
read the list.

> However AMD just recently posted a VNMI patch series to avoid
> single stepping the CPU when NMI is blocked due to the same reason, because
> it is fragile.

The vNMI feature isn't available in any shipping processor yet, is it?

> Do you really want KVM to single step the guest in this case, to deliver the #SMI?
> I can do it, but it is bound to cause lot of trouble.

Perhaps you could document this as a KVM erratum...one of many
involving virtual SMI delivery.