linux-kernel - [PATCH] KVM: X86: always reset st->preempted in record_steal

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Date:   Fri, 28 May 2021 01:33:58 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Oliver Upton <oupton@...gle.com>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Peter Shier <pshier@...gle.com>, kvm@...r.kernel.org
Subject: [PATCH] KVM: X86: always reset st->preempted in record_steal_time()

From: Lai Jiangshan <laijs@...ux.alibaba.com>

st->preempted needs to be reset in record_steal_time() to clear the
KVM_VCPU_PREEMPTED bit.

But the commit 66570e966dd9 ("kvm: x86: only provide PV features if
enabled in guest's CPUID") made it cleared conditionally and
KVM_VCPU_PREEMPTED might not be cleared when entering into the guest.

Also make st->preempted be only read once, so that trace_kvm_pv_tlb_flush()
and kvm_vcpu_flush_tlb_guest() is consistent with same value of st->preempted.

Cc: Oliver Upton <oupton@...gle.com>
Fixes: 66570e966dd9 ("kvm: x86: only provide PV features if enabled in guest's CPUID")
Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
---
 arch/x86/kvm/x86.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index bbc4e04e67ad..b8a7259ebd14 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3101,10 +3101,14 @@ 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;
 	}
 
 	vcpu->arch.st.preempted = 0;
-- 
2.19.1.6.gb485710b