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

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Message-ID: <1a4f35e356c50e38916acef6c86175b24efca0a3.camel@redhat.com>
Date:   Mon, 15 Mar 2021 20:19:09 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...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 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
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.

What do you think?

> 
> This also makes the code more readable since you can write
> 
> 	if (guest_cpuid_is_intel(vcpu)) {
> 		/*
> 		 * 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.
> 		 */
> 		...
> 	} else {
> 		/* Do the opposite.  */
> 		...
> 	}

Best regards,
	Maxim Levitsky

> 
> Paolo
>