KVM: X86: Fix vCPU preempted state from guest's point of view

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@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Message-Id: <1621339235-11131-3-git-send-email-wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7f767d5..1090416 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3006,6 +3006,8 @@ 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;