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

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Date:   Wed, 22 Feb 2023 14:49:31 +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
Subject: Re: [PATCH 08/12] KVM: nSVM: Use KVM-governed feature framework to
 track "vVM{SAVE,LOAD} enabled"

On Tue, Feb 21, 2023 at 03:48:07PM -0800, Sean Christopherson wrote:
> On Tue, Feb 21, 2023, Yu Zhang wrote:
> > > Sorry, why guest_cpuid_is_intel(vcpu)? Is it becasue that a AMD host with virtual
> > > VMSAVE/VMLOAD capability will always expose this feature for all AMD guests? 
> > 
> > Oh, sorry. I missed the guest_cpuid_has() in kvm_governed_feature_check_and_set().
> > So please just ignore my 2nd question.
> > 
> > As to the check of guest_cpuid_is_intel(), is it necessary?
> 
> Yes?  The comment in init_vmcb_after_set_cpuid() says:
> 
> 		/*
> 		 * We must intercept SYSENTER_EIP and SYSENTER_ESP
> 		 * accesses because the processor only stores 32 bits.
> 		 * For the same reason we cannot use virtual VMLOAD/VMSAVE.
> 		 */
> 
> but I'm struggling to connect the dots to SYSENTER.  I suspect the comment is
> misleading and has nothing to do 32-bit vs. 64-bit (or I'm reading it wrong) and
> should be something like:
> 
> 	/*
> 	 * Disable virtual VMLOAD/VMSAVE and intercept VMLOAD/VMSAVE if the
> 	 * guest CPU is Intel in order to inject #UD.
> 	 */
> 
> In other words, a non-SVM guest shouldn't be allowed to execute VMLOAD/VMSAVE.

Yes. Such interpretation makes sense. And vmload/vmsave shall be intercepted
if guest CPU is Intel and #UD shall be injected. I guess this is done indirectly
by judging the EFER_SVME not set in EFER in nested_svm_check_permissions()?

And as to X86_FEATURE_V_VMSAVE_VMLOAD, should the guest_cpuid_has() return true
at all for a Intel guest?

B.R.
Yu