[PATCH] KVM: X86: correct meaningless kvm_apicv_activated() check

[PATCH] KVM: X86: correct meaningless kvm_apicv_activated() check

[Paolo Bonzini]

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

Re: [PATCH] KVM: X86: correct meaningless kvm_apicv_activated() check

[Vitaly Kuznetsov]

Paolo Bonzini <pbonzini@redhat.com> 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@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);

'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@redhat.com>

-- 
Vitaly

Re: [PATCH] KVM: X86: correct meaningless kvm_apicv_activated() check

[Sean Christopherson]

On Mon, Mar 16, 2020 at 09:33:50AM +0100, Vitaly Kuznetsov wrote:
> Paolo Bonzini <pbonzini@redhat.com> writes:
> > +	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 :-)

Or maybe this to avoid so many equals signs?

	if (!old == !new)
		return;

Re: [PATCH] KVM: X86: correct meaningless kvm_apicv_activated() check

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> On Mon, Mar 16, 2020 at 09:33:50AM +0100, Vitaly Kuznetsov wrote:
>> Paolo Bonzini <pbonzini@redhat.com> writes:
>> > +	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 :-)
>
> Or maybe this to avoid so many equals signs?
>
> 	if (!old == !new)
> 		return;
>

	if (!!old == !!new)
		return;

to make it clear we're converting them to 1/0 :-)

-- 
Vitaly

Re: [PATCH] KVM: X86: correct meaningless kvm_apicv_activated() check

[Sean Christopherson]

On Mon, Mar 16, 2020 at 04:44:47PM +0100, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@intel.com> writes:
> 
> > On Mon, Mar 16, 2020 at 09:33:50AM +0100, Vitaly Kuznetsov wrote:
> >> Paolo Bonzini <pbonzini@redhat.com> writes:
> >> > +	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 :-)
> >
> > Or maybe this to avoid so many equals signs?
> >
> > 	if (!old == !new)
> > 		return;
> >
> 
> 	if (!!old == !!new)
> 		return;
> 
> to make it clear we're converting them to 1/0 :-)

All I can think of now is the Onion article regarding razor blades...

	if (!!!!old == !!!!new)
		return;

Re: [PATCH] KVM: X86: correct meaningless kvm_apicv_activated() check

[Paolo Bonzini]

On 16/03/20 16:59, Sean Christopherson wrote:
>>>
>> 	if (!!old == !!new)
>> 		return;
>>
>> to make it clear we're converting them to 1/0 :-)
>
> All I can think of now is the Onion article regarding razor blades...
> 
> 	if (!!!!old == !!!!new)
> 		return;
> 

That would be !!!!!, but seriously I'll go with two.

(Thanks for giving me a chuckle, it's sorely needed these days).

Paolo

Re: [PATCH] KVM: X86: correct meaningless kvm_apicv_activated() check

[Xiaoyao Li]

On 3/17/2020 12:39 AM, Paolo Bonzini wrote:
> On 16/03/20 16:59, Sean Christopherson wrote:
>>>>
>>> 	if (!!old == !!new)
>>> 		return;
>>>
>>> to make it clear we're converting them to 1/0 :-)
>>
>> All I can think of now is the Onion article regarding razor blades...
>>
>> 	if (!!!!old == !!!!new)
>> 		return;
>>
> 
> That would be !!!!!, but seriously I'll go with two.
> 
> (Thanks for giving me a chuckle, it's sorely needed these days).

Take care, Paolo.

I have been staying at home for two months in Wuhan, China, and things 
are going better now. I believe all the world can defeat Coronavirus 
eventually.

> Paolo
>