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From:   Lai Jiangshan <jiangshanlai@gmail.com>
To:     linux-kernel@vger.kernel.org
Cc:     Lai Jiangshan <laijs@linux.alibaba.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        kvm@vger.kernel.org
Subject: [PATCH] KVM: X86: reset and read st->preempted in atomic way
Date:   Tue,  1 Jun 2021 01:46:28 +0800
Message-Id: <20210531174628.10265-1-jiangshanlai@gmail.com>
In-Reply-To: <CANRm+CypKbrhwFR-jLCuUruXwApq4Tb62U_KP_4H-_=7yX1VQg@mail.gmail.com>
References: <CANRm+CypKbrhwFR-jLCuUruXwApq4Tb62U_KP_4H-_=7yX1VQg@mail.gmail.com>
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From: Lai Jiangshan <laijs@linux.alibaba.com>

In record_steal_time(), st->preempted is read twice, and
trace_kvm_pv_tlb_flush() might output result inconsistent if
kvm_vcpu_flush_tlb_guest() see a different st->preempted later.

It is a very trivial problem and hardly has actual harm and can be
avoided by reseting and reading st->preempted in atomic way via xchg().

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0087d3532c98..fba39fe162da 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3117,9 +3117,11 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
 	 * expensive IPIs.
 	 */
 	if (guest_pv_has(vcpu, KVM_FEATURE_PV_TLB_FLUSH)) {
+		u8 st_preempted = xchg(&st->preempted, 0);
+
 		trace_kvm_pv_tlb_flush(vcpu->vcpu_id,
-				       st->preempted & KVM_VCPU_FLUSH_TLB);
-		if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
+				       st_preempted & KVM_VCPU_FLUSH_TLB);
+		if (st_preempted & KVM_VCPU_FLUSH_TLB)
 			kvm_vcpu_flush_tlb_guest(vcpu);
 	} else {
 		st->preempted = 0;
-- 
2.19.1.6.gb485710b