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Subject: Re: [PATCH RFC] KVM: nSVM: Fix L1 state corruption upon return from
 SMM
To:     Sean Christopherson <seanjc@google.com>,
        Maxim Levitsky <mlevitsk@redhat.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Cathy Avery <cavery@redhat.com>,
        Emanuele Giuseppe Esposito <eesposit@redhat.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <82327cd1-92ca-9f6b-3af0-8215e9d21eae@redhat.com>
Date:   Wed, 23 Jun 2021 22:37:53 +0200
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On 23/06/21 18:21, Sean Christopherson wrote:
> On Wed, Jun 23, 2021, Sean Christopherson wrote:
>> And I believe this hackery is necessary only because nested_svm_vmexit() isn't
>> following the architcture in the first place.  I.e. using vmcb01 to restore
>> host state is flat out wrong.
> 
> Ah, that's not true, using vmcb01 is allowed by "may store some or all host state
> in hidden on-chip memory".

And also, "Different implementations may choose to save the hidden parts 
of the host’s segment registers as well as the selectors".

>  From a performance perspective, I do like the SMI/RSM shenanigans.  I'm not
> totally opposed to the trickery since I think it will break a guest if and only
> if the L1 guest is also violating the APM.  And we're not fudging the spec thaat
> much :-)

Yeah, that was my reasoning as well.  Any reference to "hidden on-chip 
memory", plus the forbidding modifications of the host save area, sort 
of implies that the processor can actually flush that hidden on-chip 
memory for whatever reason (such as on some sleep states?!?).

Paolo