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

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Date:   Wed, 29 Jun 2022 09:31:12 -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, Jun 21, 2022 at 8:09 AM Maxim Levitsky <mlevitsk@...hat.com> wrote:
>
> When #SMI is asserted, the CPU can be in interrupt shadow
> due to sti or mov ss.
>
> It is not mandatory in  Intel/AMD prm to have the #SMI
> blocked during the shadow, and on top of
> that, since neither SVM nor VMX has true support for SMI
> window, waiting for one instruction would mean single stepping
> the guest.
>
> Instead, allow #SMI in this case, but both reset the interrupt
> window and stash its value in SMRAM to restore it on exit
> from SMM.
>
> This fixes rare failures seen mostly on windows guests on VMX,
> when #SMI falls on the sti instruction which mainfest in
> VM entry failure due to EFLAGS.IF not being set, but STI interrupt
> window still being set in the VMCS.

I think you're just making stuff up! See Note #5 at
https://sandpile.org/x86/inter.htm.

Can you reference the vendors' documentation that supports this change?