linux-kernel - Re: [PATCH 2/2] KVM: nSVM: improve SYSENTER emulation on AMD

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Date:   Tue, 16 Mar 2021 09:16:48 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 2/2] KVM: nSVM: improve SYSENTER emulation on AMD

On 15/03/21 19:19, Maxim Levitsky wrote:
> On Mon, 2021-03-15 at 18:56 +0100, Paolo Bonzini wrote:
>> On 15/03/21 18:43, Maxim Levitsky wrote:
>>> +	if (!guest_cpuid_is_intel(vcpu)) {
>>> +		/*
>>> +		 * If hardware supports Virtual VMLOAD VMSAVE then enable it
>>> +		 * in VMCB and clear intercepts to avoid #VMEXIT.
>>> +		 */
>>> +		if (vls) {
>>> +			svm_clr_intercept(svm, INTERCEPT_VMLOAD);
>>> +			svm_clr_intercept(svm, INTERCEPT_VMSAVE);
>>> +			svm->vmcb->control.virt_ext |= VIRTUAL_VMLOAD_VMSAVE_ENABLE_MASK;
>>> +		}
>>> +		/* No need to intercept these msrs either */
>>> +		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SYSENTER_EIP, 1, 1);
>>> +		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SYSENTER_ESP, 1, 1);
>>> +	}
>>
>> An "else" is needed here to do the opposite setup (removing the "if
>> (vls)" from init_vmcb).
> 
> init_vmcb currently set the INTERCEPT_VMLOAD and INTERCEPT_VMSAVE and it doesn't enable vls

There's also this towards the end of the function:

         /*
          * If hardware supports Virtual VMLOAD VMSAVE then enable it
          * in VMCB and clear intercepts to avoid #VMEXIT.
          */
         if (vls) {
                 svm_clr_intercept(svm, INTERCEPT_VMLOAD);
                 svm_clr_intercept(svm, INTERCEPT_VMSAVE);
                 svm->vmcb->control.virt_ext |= 
VIRTUAL_VMLOAD_VMSAVE_ENABLE_MASK;
         }

> thus there is nothing to do if I don't want to enable vls.
> It seems reasonable to me.
> 
> Both msrs I marked as '.always = false' in the
> 'direct_access_msrs', which makes them be intercepted by the default.
> If I were to use '.always = true' it would feel a bit wrong as the intercept is not always
> enabled.

I agree that .always = false is correct.

> What do you think?

You can set the CPUID multiple times, so you could go from AMD to Intel 
and back.

Thanks,

Paolo