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Subject[PATCH] KVM: X86: correct meaningless kvm_apicv_activated() check
Date
After test_and_set_bit() for kvm->arch.apicv_inhibit_reasons, we will
always get false when calling kvm_apicv_activated() because it's sure
apicv_inhibit_reasons do not equal to 0.

What the code wants to do, is check whether APICv was *already* active
and if so skip the costly request; we can do this using cmpxchg.

Reported-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/kvm/x86.c | 25 ++++++++++++++++---------
1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a7cb85231330..49efa4529662 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8049,19 +8049,26 @@ void kvm_vcpu_update_apicv(struct kvm_vcpu *vcpu)
*/
void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
{
+ unsigned long old, new, expected;
+
if (!kvm_x86_ops->check_apicv_inhibit_reasons ||
!kvm_x86_ops->check_apicv_inhibit_reasons(bit))
return;

- if (activate) {
- if (!test_and_clear_bit(bit, &kvm->arch.apicv_inhibit_reasons) ||
- !kvm_apicv_activated(kvm))
- return;
- } else {
- if (test_and_set_bit(bit, &kvm->arch.apicv_inhibit_reasons) ||
- kvm_apicv_activated(kvm))
- return;
- }
+ old = READ_ONCE(kvm->arch.apicv_inhibit_reasons);
+ do {
+ expected = new = old;
+ if (activate)
+ __clear_bit(bit, &new);
+ else
+ __set_bit(bit, &new);
+ if (new == old)
+ break;
+ old = cmpxchg(&kvm->arch.apicv_inhibit_reasons, expected, new);
+ } while (old != expected);
+
+ if ((old == 0) == (new == 0))
+ return;

trace_kvm_apicv_update_request(activate, bit);
if (kvm_x86_ops->pre_update_apicv_exec_ctrl)
--
1.8.3.1
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