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Subject: Re: [PATCH v2 00/21] KVM: x86: Event/exception fixes and cleanups
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Sean Christopherson <seanjc@google.com>,
        Jim Mattson <jmattson@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Oliver Upton <oupton@google.com>,
        Peter Shier <pshier@google.com>
Date:   Wed, 06 Jul 2022 23:03:53 +0300
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On Wed, 2022-07-06 at 17:52 +0000, Sean Christopherson wrote:
> On Wed, Jul 06, 2022, Jim Mattson wrote:
> > On Wed, Jul 6, 2022 at 4:55 AM Maxim Levitsky <mlevitsk@redhat.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.
> 

For preserving pending MTF, I guess it makes sense to use vmcb12, especially since we own
its format.

Best regards,
	Maxim Levitsky