linux-kernel - [PATCH 06/14] KVM: x86: SVM: don't set VMLOAD/VMSAVE intercepts on vCPU reset

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Date:   Tue, 14 Sep 2021 18:48:17 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>, Bandan Das <bsd@...hat.com>,
        linux-kernel@...r.kernel.org (open list),
        Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...hat.com>,
        Wei Huang <wei.huang2@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kselftest@...r.kernel.org (open list:KERNEL SELFTEST FRAMEWORK),
        Maxim Levitsky <mlevitsk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        x86@...nel.org (maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)),
        Shuah Khan <shuah@...nel.org>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: [PATCH 06/14] KVM: x86: SVM: don't set VMLOAD/VMSAVE intercepts on vCPU reset

commit adc2a23734ac ("KVM: nSVM: improve SYSENTER emulation on AMD"),
made init_vmcb set vmload/vmsave intercepts unconditionally,
and relied on svm_vcpu_after_set_cpuid to clear them when possible.

However init_vmcb is also called when the vCPU is reset, and it is
not followed by another call to svm_vcpu_after_set_cpuid because
the CPUID is already set.

This mistake makes the VMSAVE/VMLOAD intercept to be set when
it is not needed, and harms performance of the nested guest.

Fixes: adc2a23734ac ("KVM: nSVM: improve SYSENTER emulation on AMD")

Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
---
 arch/x86/kvm/svm/svm.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 6645542df9bd..861ac9f74331 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1199,8 +1199,6 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
 	svm_set_intercept(svm, INTERCEPT_SHUTDOWN);
 	svm_set_intercept(svm, INTERCEPT_VMRUN);
 	svm_set_intercept(svm, INTERCEPT_VMMCALL);
-	svm_set_intercept(svm, INTERCEPT_VMLOAD);
-	svm_set_intercept(svm, INTERCEPT_VMSAVE);
 	svm_set_intercept(svm, INTERCEPT_STGI);
 	svm_set_intercept(svm, INTERCEPT_CLGI);
 	svm_set_intercept(svm, INTERCEPT_SKINIT);
@@ -1377,6 +1375,10 @@ static int svm_create_vcpu(struct kvm_vcpu *vcpu)
 	svm->guest_state_loaded = false;
 
 	svm_switch_vmcb(svm, &svm->vmcb01);
+
+	svm_set_intercept(svm, INTERCEPT_VMLOAD);
+	svm_set_intercept(svm, INTERCEPT_VMSAVE);
+
 	init_vmcb(vcpu);
 
 	svm_vcpu_init_msrpm(vcpu, svm->msrpm);
-- 
2.26.3