[PATCH 5.12 070/296] KVM: X86: Fix vCPU preempted state from guests point of view

[Greg Kroah-Hartman]

From: Wanpeng Li <wanpengli@xxxxxxxxxxx>

commit 1eff0ada88b48e4ac1e3fe26483b3684fedecd27 upstream.

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@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
Message-Id: <1621339235-11131-3-git-send-email-wanpengli@xxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/kvm/x86.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3015,6 +3015,8 @@ static void record_steal_time(struct kvm
 				       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;