linux-kernel - Re: [PATCH 08/12] KVM: nSVM: Use KVM-governed feature framework to track "vVM{SAVE,LOAD} enabled"

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Date:   Fri, 30 Jun 2023 18:00:01 +0800
From:   Yu Zhang <yu.c.zhang@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 08/12] KVM: nSVM: Use KVM-governed feature framework to
 track "vVM{SAVE,LOAD} enabled"

On Thu, Jun 29, 2023 at 09:50:34AM -0700, Sean Christopherson wrote:
> On Wed, Feb 22, 2023, Sean Christopherson wrote:
> > +Maxim
> > 
> > On Wed, Feb 22, 2023, Yu Zhang wrote:
> > I'll opportunistically massage the comment to make it more explicit about why
> > VMLOAD needs to be intercepted.
> >  
> > That said, clearing the bits for this seems wrong.  That would corrupt the MSRs
> > for 64-bit Intel guests.  The "target" of the fix was 32-bit L2s, i.e. I doubt
> > anything would notice.
> > 
> >     This patch fixes nested migration of 32 bit nested guests, that was
> >     broken because incorrect cached values of SYSENTER msrs were stored in
> >     the migration stream if L1 changed these msrs with
> >     vmload prior to L2 entry.
> 
> Aha!  Finally figured out what this code is doing.  KVM intercepts VMLOAD so that
> KVM can correctly model the VMLOAD behavior of dropping bits 63:32, i.e. to clear
> svm->sysenter_eip_hi and svm->sysenter_esp_hi.
> 
> So the code is correct.  I'll add this comment:
> 
> 	/*
> 	 * Intercept VMLOAD if the vCPU mode is Intel in order to emulate that
> 	 * VMLOAD drops bits 63:32 of SYSENTER (ignoring the fact that exposing
> 	 * SVM on Intel is bonkers and extremely unlikely to work).
> 	 */
> 
Oh.. Because L2 will never be a 64-bit Intel guest, and the emulation of vmload
shall follow APM's requirement(to clear the upper 32 bits)?

Thanks a lot for bring me back to this discussion... I totally forgot it. :)

B.R.
Yu
 
Thanks a lot for this explanation, Sean!