[PATCH] KVM: apic: reuse smp_wmb() in kvm_make_request()

[PATCH] KVM: apic: reuse smp_wmb() in kvm_make_request()

[linmiaohe]

From: Miaohe Lin <linmiaohe@huawei.com>

There is already an smp_mb() barrier in kvm_make_request(). We reuse it
here.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 arch/x86/kvm/lapic.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index eafc631d305c..ea871206a370 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1080,9 +1080,12 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
 			result = 1;
 			/* assumes that there are only KVM_APIC_INIT/SIPI */
 			apic->pending_events = (1UL << KVM_APIC_INIT);
-			/* make sure pending_events is visible before sending
-			 * the request */
-			smp_wmb();
+			/*
+			 * Make sure pending_events is visible before sending
+			 * the request.
+			 * There is already an smp_wmb() in kvm_make_request(),
+			 * we reuse that barrier here.
+			 */
 			kvm_make_request(KVM_REQ_EVENT, vcpu);
 			kvm_vcpu_kick(vcpu);
 		}
-- 
2.19.1

Re: [PATCH] KVM: apic: reuse smp_wmb() in kvm_make_request()

[Vitaly Kuznetsov]

linmiaohe <linmiaohe@huawei.com> writes:

> From: Miaohe Lin <linmiaohe@huawei.com>
>
> There is already an smp_mb() barrier in kvm_make_request(). We reuse it
> here.
>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  arch/x86/kvm/lapic.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index eafc631d305c..ea871206a370 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1080,9 +1080,12 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
>  			result = 1;
>  			/* assumes that there are only KVM_APIC_INIT/SIPI */
>  			apic->pending_events = (1UL << KVM_APIC_INIT);
> -			/* make sure pending_events is visible before sending
> -			 * the request */
> -			smp_wmb();
> +			/*
> +			 * Make sure pending_events is visible before sending
> +			 * the request.
> +			 * There is already an smp_wmb() in kvm_make_request(),
> +			 * we reuse that barrier here.
> +			 */

Let me suggest an alternative wording,

"kvm_make_request() provides smp_wmb() so pending_events changes are
guaranteed to be visible"

But there is nothing wrong with yours, it's just longer than it could be
:-)

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

>  			kvm_make_request(KVM_REQ_EVENT, vcpu);
>  			kvm_vcpu_kick(vcpu);
>  		}

-- 
Vitaly

Re: [PATCH] KVM: apic: reuse smp_wmb() in kvm_make_request()

[Sean Christopherson]

On Thu, Feb 06, 2020 at 11:47:02AM +0100, Vitaly Kuznetsov wrote:
> linmiaohe <linmiaohe@huawei.com> writes:
> 
> > From: Miaohe Lin <linmiaohe@huawei.com>
> >
> > There is already an smp_mb() barrier in kvm_make_request(). We reuse it
> > here.
> >
> > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> > ---
> >  arch/x86/kvm/lapic.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index eafc631d305c..ea871206a370 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -1080,9 +1080,12 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
> >  			result = 1;
> >  			/* assumes that there are only KVM_APIC_INIT/SIPI */
> >  			apic->pending_events = (1UL << KVM_APIC_INIT);
> > -			/* make sure pending_events is visible before sending
> > -			 * the request */
> > -			smp_wmb();
> > +			/*
> > +			 * Make sure pending_events is visible before sending
> > +			 * the request.
> > +			 * There is already an smp_wmb() in kvm_make_request(),
> > +			 * we reuse that barrier here.
> > +			 */
> 
> Let me suggest an alternative wording,
> 
> "kvm_make_request() provides smp_wmb() so pending_events changes are
> guaranteed to be visible"
> 
> But there is nothing wrong with yours, it's just longer than it could be
> :-)

I usually lean in favor of more comments, but in thise case I'd vote to
drop the comment altogether.  There are lots of places that rely on the
smp_wmb() in kvm_make_request() without a comment, e.g. the cases for
APIC_DM_STARTUP and APIC_DM_REMRD in this same switch, kvm_inject_nmi(),
etc...  One might wonder what makes INIT special.

And on the flip side, APIC_DM_STARTUP is a good example of when a
smp_wmb()/smp_rmb() is needed and commented correctly (though calling out
the exactly location of the other half would be helpful).

> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> 
> >  			kvm_make_request(KVM_REQ_EVENT, vcpu);
> >  			kvm_vcpu_kick(vcpu);
> >  		}
> 
> -- 
> Vitaly
> 

Re: [PATCH] KVM: apic: reuse smp_wmb() in kvm_make_request()

[linmiaohe]

Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> On Thu, Feb 06, 2020 at 11:47:02AM +0100, Vitaly Kuznetsov wrote:
>> linmiaohe <linmiaohe@huawei.com> writes:
>> 
>> > From: Miaohe Lin <linmiaohe@huawei.com>
>> >
>> > There is already an smp_mb() barrier in kvm_make_request(). We reuse 
>> > it here.
>> > +			/*
>> > +			 * Make sure pending_events is visible before sending
>> > +			 * the request.
>> > +			 * There is already an smp_wmb() in kvm_make_request(),
>> > +			 * we reuse that barrier here.
>> > +			 */
>> 
>> Let me suggest an alternative wording,
>> 
>> "kvm_make_request() provides smp_wmb() so pending_events changes are 
>> guaranteed to be visible"
>> 
>> But there is nothing wrong with yours, it's just longer than it could 
>> be
>> :-)

Thanks for your alternative wording. It looks much better.

>I usually lean in favor of more comments, but in thise case I'd vote to drop the comment altogether.  There are lots of places that rely on the
>smp_wmb() in kvm_make_request() without a comment, e.g. the cases for APIC_DM_STARTUP and APIC_DM_REMRD in this same switch, kvm_inject_nmi(), etc...  One might wonder what makes INIT special.
>
>And on the flip side, APIC_DM_STARTUP is a good example of when a
>smp_wmb()/smp_rmb() is needed and commented correctly (though calling out the exactly location of the other half would be helpful).

Yeh, I think the comment should be dropped too. :)

Thanks to both for review! I would send v2.