linux-kernel - [PATCH v2] KVM: VMX: Micro-optimize vmexit time when not exposing PMU

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Message-Id: <1584071718-17163-1-git-send-email-wanpengli@tencent.com>
Date:   Fri, 13 Mar 2020 11:55:18 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: [PATCH v2] KVM: VMX: Micro-optimize vmexit time when not exposing PMU

From: Wanpeng Li <wanpengli@...cent.com>

PMU is not exposed to guest by most of products from cloud providers since the 
bad performance of PMU emulation and security concern. However, it calls 
perf_guest_switch_get_msrs() and clear_atomic_switch_msr() unconditionally 
even if PMU is not exposed to the guest before each vmentry. 

~2% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my 
SKX server.

Before patch:
vmcall 1559

After patch:
vmcall 1529

Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
---
v1 -> v2:
 * move the check before atomic_switch_perf_msrs

 arch/x86/kvm/vmx/vmx.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 40b1e61..b20423c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6567,7 +6567,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
 
 	pt_guest_enter(vmx);
 
-	atomic_switch_perf_msrs(vmx);
+	if (vcpu_to_pmu(vcpu)->version)
+		atomic_switch_perf_msrs(vmx);
 	atomic_switch_umwait_control_msr(vmx);
 
 	if (enable_preemption_timer)
-- 
2.7.4