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

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Message-ID: <0dbcff57-8197-8fbb-809d-b47a4f5e9e77@redhat.com>
Date:   Mon, 15 Mar 2021 18:56:00 +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 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).

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.  */
		...
	}

Paolo