Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
> 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@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> 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);
'expected' here is a bit confusing as it's not what we expect to get as
the result but rather what we expect to see pre-change. I don't have a
better suggestion though.
> +
> + if ((old == 0) == (new == 0))
> + return;
This is a very laconic expression I personally find hard to read :-)
/* Check if WE actually changed APICv state */
if ((!old && !new) || (old && new))
return;
would be my preference (not strong though, I read yours several times
and now I feel like I understand it just fine :-)
>
> trace_kvm_apicv_update_request(activate, bit);
> if (kvm_x86_ops->pre_update_apicv_exec_ctrl)
Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
--
Vitaly
