linux-kernel - [PATCH] KVM: X86: Properly account for guest CPU time when considering context tracking

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Date:   Mon, 29 Mar 2021 17:43:56 +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 <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: [PATCH] KVM: X86: Properly account for guest CPU time when considering context tracking

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

The bugzilla https://bugzilla.kernel.org/show_bug.cgi?id=209831 
reported that the guest time remains 0 when running a while true 
loop in the guest.

The commit 87fa7f3e98a131 ("x86/kvm: Move context tracking where it 
belongs") moves guest_exit_irqoff() close to vmexit breaks the 
tick-based time accouting when the ticks that happen after IRQs are 
disabled are incorrectly accounted to the host/system time. This is 
because we exit the guest state too early.

vtime-based time accounting is tied to context tracking, keep the 
guest_exit_irqoff() around vmexit code when both vtime-based time 
accounting and specific cpu is context tracking mode active. 
Otherwise, leave guest_exit_irqoff() after handle_exit_irqoff() 
and explicit IRQ window for tick-based time accouting.

Fixes: 87fa7f3e98a131 ("x86/kvm: Move context tracking where it belongs")
Cc: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
---
 arch/x86/kvm/svm/svm.c | 3 ++-
 arch/x86/kvm/vmx/vmx.c | 3 ++-
 arch/x86/kvm/x86.c     | 2 ++
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 58a45bb..55fb5ce 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3812,7 +3812,8 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 	 * into world and some more.
 	 */
 	lockdep_hardirqs_off(CALLER_ADDR0);
-	guest_exit_irqoff();
+	if (vtime_accounting_enabled_this_cpu())
+		guest_exit_irqoff();
 
 	instrumentation_begin();
 	trace_hardirqs_off_finish();
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 32cf828..85695b3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6689,7 +6689,8 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 	 * into world and some more.
 	 */
 	lockdep_hardirqs_off(CALLER_ADDR0);
-	guest_exit_irqoff();
+	if (vtime_accounting_enabled_this_cpu())
+		guest_exit_irqoff();
 
 	instrumentation_begin();
 	trace_hardirqs_off_finish();
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fe806e8..234c8b3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9185,6 +9185,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	++vcpu->stat.exits;
 	local_irq_disable();
 	kvm_after_interrupt(vcpu);
+	if (!vtime_accounting_enabled_this_cpu())
+		guest_exit_irqoff();
 
 	if (lapic_in_kernel(vcpu)) {
 		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;
-- 
2.7.4