linux-kernel - [PATCH v2 3/4] KVM: X86: Fix vCPU preempted state from guest's point of view

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Message-Id: <1620871189-4763-2-git-send-email-wanpengli@tencent.com>
Date:   Thu, 13 May 2021 09:59:48 +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>, stable@...r.kernel.org
Subject: [PATCH v2 3/4] KVM: X86: Fix vCPU preempted state from guest's point of view

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

Commit 66570e966dd9 (kvm: x86: only provide PV features if enabled in guest's 
CPUID) avoids to access pv tlb shootdown host side logic when this pv feature 
is not exposed to guest, however, kvm_steal_time.preempted not only leveraged 
by pv tlb shootdown logic but also mitigate the lock holder preemption issue. 
>From guest's point of view, vCPU is always preempted since we lose the reset 
of kvm_steal_time.preempted before vmentry if pv tlb shootdown feature is not 
exposed. This patch fixes it by clearing kvm_steal_time.preempted before 
vmentry.

Fixes: 66570e966dd9 (kvm: x86: only provide PV features if enabled in guest's CPUID)
Reviewed-by: Sean Christopherson <seanjc@...gle.com>
Cc: stable@...r.kernel.org
Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
---
 arch/x86/kvm/x86.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index dfb7c32..bed7b53 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3105,6 +3105,8 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
 				       st->preempted & KVM_VCPU_FLUSH_TLB);
 		if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
 			kvm_vcpu_flush_tlb_guest(vcpu);
+	} else {
+		st->preempted = 0;
 	}
 
 	vcpu->arch.st.preempted = 0;
-- 
2.7.4