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SubjectRe: [PATCH] KVM: X86: avoid meaningless kvm_apicv_activated() check
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linmiaohe <linmiaohe@huawei.com> writes:

> From: Miaohe Lin <linmiaohe@huawei.com>
>
> 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.
>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
> arch/x86/kvm/x86.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ddcc51b89e2c..fa62dcb0ed0c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8018,8 +8018,7 @@ void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
> !kvm_apicv_activated(kvm))
> return;
> } else {
> - if (test_and_set_bit(bit, &kvm->arch.apicv_inhibit_reasons) ||
> - kvm_apicv_activated(kvm))
> + if (test_and_set_bit(bit, &kvm->arch.apicv_inhibit_reasons))
> return;
> }

This seems to be correct in a sense that we are not really protected
against concurrent modifications of 'apicv_inhibit_reasons' (like what
if 'apicv_inhibit_reasons' gets modified right after we've checked
'kvm_apicv_activated(kvm)').

The function, however, still gives a flase impression it is somewhat
protected against concurent modifications. Like what are these
test_and_{set,clear}_bit() for?

If I'm not mistaken, the logic this function was supposed to implement
is: change the requested bit to the requested state and, if
kvm_apicv_activated() changed (we set the first bit or cleared the
last), proceed with KVM_REQ_APICV_UPDATE. What if we re-write it like

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2103101eca78..b97b8ff4a789 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8027,19 +8027,19 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_update_apicv);
*/
void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
{
+ bool apicv_was_activated = kvm_apicv_activated(kvm);
+
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;
- }
+ if (activate)
+ clear_bit(bit, &kvm->arch.apicv_inhibit_reasons);
+ else
+ set_bit(bit, &kvm->arch.apicv_inhibit_reasons);
+
+ if (kvm_apicv_activated(kvm) == apicv_was_activated)
+ return;

trace_kvm_apicv_update_request(activate, bit);
if (kvm_x86_ops->pre_update_apicv_exec_ctrl)
Is this equal?

--
Vitaly

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