linux-kernel - Re: [PATCH v2 00/21] KVM: x86: Event/exception fixes and cleanups

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Date:   Wed, 6 Jul 2022 17:52:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH v2 00/21] KVM: x86: Event/exception fixes and cleanups

On Wed, Jul 06, 2022, Jim Mattson wrote:
> On Wed, Jul 6, 2022 at 4:55 AM Maxim Levitsky <mlevitsk@...hat.com> wrote:
> 
> > 1. Since #SMI is higher priority than the #MTF, that means that unless dual monitor treatment is used,
> >    and the dual monitor handler figures out that #MTF was pending and re-injects it when it
> >    VMRESUME's the 'host', the MTF gets lost, and there is no way for a normal hypervisor to
> >    do anything about it.
> >
> >    Or maybe pending MTF is saved to SMRAM somewhere.
> >
> >    In case you will say that I am inventing this again, I am saying now that the above is
> >    just a guess.
> 
> This is covered in the SDM, volume 3, section 31.14.1: "Default
> Treatment of SMI Delivery:"
> 
> The pseudocode above makes reference to the saving of VMX-critical
> state. This state consists of the following:
> (1) SS.DPL (the current privilege level); (2) RFLAGS.VM2; (3) the
> state of blocking by STI and by MOV SS (see
> Table 24-3 in Section 24.4.2); (4) the state of virtual-NMI blocking
> (only if the processor is in VMX non-root oper-
> ation and the “virtual NMIs” VM-execution control is 1); and (5) an
> indication of whether an MTF VM exit is pending
> (see Section 25.5.2). These data may be saved internal to the
> processor or in the VMCS region of the current
> VMCS. Processors that do not support SMI recognition while there is
> blocking by STI or by MOV SS need not save
> the state of such blocking.
> 
> Saving VMX-critical state to SMRAM is not documented as an option.

Hmm, I'm not entirely convinced that Intel doesn't interpret "internal to the
processor" as "undocumented SMRAM fields".  But I could also be misremembering
the SMI flows.

Regardless, I do like the idea of using vmcs12 instead of SMRAM.  That would provide
some extra motivation for moving away from KVM's broken pseudo VM-Exit implementation.