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

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Message-Id: <1584007547-4802-1-git-send-email-wanpengli@tencent.com>
Date:   Thu, 12 Mar 2020 18:05:47 +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] 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 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. 

~1.28% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my 
SKX server.

Before patch:
vmcall 1559

After patch:
vmcall 1539

Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
---
 arch/x86/kvm/vmx/vmx.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 40b1e61..fd526c8 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6441,6 +6441,9 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
 	int i, nr_msrs;
 	struct perf_guest_switch_msr *msrs;
 
+	if (!vcpu_to_pmu(&vmx->vcpu)->version)
+		return;
+
 	msrs = perf_guest_get_msrs(&nr_msrs);
 
 	if (!msrs)
-- 
2.7.4