[PATCH 5/5] KVM: x86/hyperv: Use {READ,WRITE}_ONCE for cross-task synic->active accesses

[Sean Christopherson <seanjc@google.com>]

When activating Hyper-V's Synthetic Interrupt Controller (SynIC), mark it
active with WRITE_ONCE() and query it using READ_ONCE() in synic_get(),
the only known cross-task reader, to document that the flag is accessed
without holding the vCPU's mutex.

Note, there are no data dependencies on the SynIC being marked active,
e.g. the vector read by synic_set_irq() is set (usually in response to
guest activity) long after the SynIC is initially activated, and a false
negative on the SynIC being active would be benign (ignoring that such a
race is likely to be problematic for the guest irrespective of what KVM
does).

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/hyperv.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index a79ccea05a65..cf35ec93cffb 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -219,7 +219,7 @@ static struct kvm_vcpu_hv_synic *synic_get(struct kvm *kvm, u32 vpidx)
 		return NULL;
 
 	synic = &hv_vcpu->synic;
-	return (synic->active) ? synic : NULL;
+	return READ_ONCE(synic->active) ? synic : NULL;
 }
 
 static void kvm_hv_notify_acked_sint(struct kvm_vcpu *vcpu, u32 sint)
@@ -1013,7 +1013,7 @@ int kvm_hv_activate_synic(struct kvm_vcpu *vcpu, bool dont_zero_synic_pages)
 
 	synic = to_hv_synic(vcpu);
 
-	synic->active = true;
+	WRITE_ONCE(synic->active, true);
 	synic->dont_zero_synic_pages = dont_zero_synic_pages;
 	synic->control = HV_SYNIC_CONTROL_ENABLE;
 	return 0;
-- 
2.54.0.545.g6539524ca2-goog