[v1] KVM: s390: Couple of small cmpxchg() optimizations

[PATCH 2/3] KVM: s390: Remove one byte cmpxchg() usage

Posted by Heiko Carstens 1 year, 2 months ago

Within sca_clear_ext_call() cmpxchg() is used to clear one or two bytes
(depending on sca format). The cmpxchg() calls are not supposed to fail; if
so that would be a bug. Given that cmpxchg() usage on one and two byte
areas generates very inefficient code, replace them with block concurrent
WRITE_ONCE() calls, and remove the WARN_ON().

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
---
 arch/s390/kvm/interrupt.c | 13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index eff69018cbeb..3fd21037479f 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -118,8 +118,6 @@ static int sca_inject_ext_call(struct kvm_vcpu *vcpu, int src_id)
 
 static void sca_clear_ext_call(struct kvm_vcpu *vcpu)
 {
-	int rc, expect;
-
 	if (!kvm_s390_use_sca_entries())
 		return;
 	kvm_s390_clear_cpuflags(vcpu, CPUSTAT_ECALL_PEND);
@@ -128,23 +126,16 @@ static void sca_clear_ext_call(struct kvm_vcpu *vcpu)
 		struct esca_block *sca = vcpu->kvm->arch.sca;
 		union esca_sigp_ctrl *sigp_ctrl =
 			&(sca->cpu[vcpu->vcpu_id].sigp_ctrl);
-		union esca_sigp_ctrl old;
 
-		old = READ_ONCE(*sigp_ctrl);
-		expect = old.value;
-		rc = cmpxchg(&sigp_ctrl->value, old.value, 0);
+		WRITE_ONCE(sigp_ctrl->value, 9);
 	} else {
 		struct bsca_block *sca = vcpu->kvm->arch.sca;
 		union bsca_sigp_ctrl *sigp_ctrl =
 			&(sca->cpu[vcpu->vcpu_id].sigp_ctrl);
-		union bsca_sigp_ctrl old;
 
-		old = READ_ONCE(*sigp_ctrl);
-		expect = old.value;
-		rc = cmpxchg(&sigp_ctrl->value, old.value, 0);
+		WRITE_ONCE(sigp_ctrl->value, 0);
 	}
 	read_unlock(&vcpu->kvm->arch.sca_lock);
-	WARN_ON(rc != expect); /* cannot clear? */
 }
 
 int psw_extint_disabled(struct kvm_vcpu *vcpu)
-- 
2.45.2

Re: [PATCH 2/3] KVM: s390: Remove one byte cmpxchg() usage

Posted by Claudio Imbrenda 1 year, 2 months ago

On Mon, 25 Nov 2024 12:50:38 +0100
Heiko Carstens <hca@linux.ibm.com> wrote:

> Within sca_clear_ext_call() cmpxchg() is used to clear one or two bytes
> (depending on sca format). The cmpxchg() calls are not supposed to fail; if
> so that would be a bug. Given that cmpxchg() usage on one and two byte
> areas generates very inefficient code, replace them with block concurrent
> WRITE_ONCE() calls, and remove the WARN_ON().
> 
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
> ---
>  arch/s390/kvm/interrupt.c | 13 ++-----------
>  1 file changed, 2 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
> index eff69018cbeb..3fd21037479f 100644
> --- a/arch/s390/kvm/interrupt.c
> +++ b/arch/s390/kvm/interrupt.c
> @@ -118,8 +118,6 @@ static int sca_inject_ext_call(struct kvm_vcpu *vcpu, int src_id)
>  
>  static void sca_clear_ext_call(struct kvm_vcpu *vcpu)
>  {
> -	int rc, expect;
> -
>  	if (!kvm_s390_use_sca_entries())
>  		return;
>  	kvm_s390_clear_cpuflags(vcpu, CPUSTAT_ECALL_PEND);
> @@ -128,23 +126,16 @@ static void sca_clear_ext_call(struct kvm_vcpu *vcpu)
>  		struct esca_block *sca = vcpu->kvm->arch.sca;
>  		union esca_sigp_ctrl *sigp_ctrl =
>  			&(sca->cpu[vcpu->vcpu_id].sigp_ctrl);
> -		union esca_sigp_ctrl old;
>  
> -		old = READ_ONCE(*sigp_ctrl);
> -		expect = old.value;
> -		rc = cmpxchg(&sigp_ctrl->value, old.value, 0);
> +		WRITE_ONCE(sigp_ctrl->value, 9);

that's supposed to be a 0, right?

>  	} else {
>  		struct bsca_block *sca = vcpu->kvm->arch.sca;
>  		union bsca_sigp_ctrl *sigp_ctrl =
>  			&(sca->cpu[vcpu->vcpu_id].sigp_ctrl);
> -		union bsca_sigp_ctrl old;
>  
> -		old = READ_ONCE(*sigp_ctrl);
> -		expect = old.value;
> -		rc = cmpxchg(&sigp_ctrl->value, old.value, 0);
> +		WRITE_ONCE(sigp_ctrl->value, 0);
>  	}
>  	read_unlock(&vcpu->kvm->arch.sca_lock);
> -	WARN_ON(rc != expect); /* cannot clear? */
>  }
>  
>  int psw_extint_disabled(struct kvm_vcpu *vcpu)

Re: [PATCH 2/3] KVM: s390: Remove one byte cmpxchg() usage

Posted by Heiko Carstens 1 year, 2 months ago

On Mon, Nov 25, 2024 at 01:16:17PM +0100, Claudio Imbrenda wrote:
> On Mon, 25 Nov 2024 12:50:38 +0100
> Heiko Carstens <hca@linux.ibm.com> wrote:
> > @@ -128,23 +126,16 @@ static void sca_clear_ext_call(struct kvm_vcpu *vcpu)
> >  		struct esca_block *sca = vcpu->kvm->arch.sca;
> >  		union esca_sigp_ctrl *sigp_ctrl =
> >  			&(sca->cpu[vcpu->vcpu_id].sigp_ctrl);
> > -		union esca_sigp_ctrl old;
> >  
> > -		old = READ_ONCE(*sigp_ctrl);
> > -		expect = old.value;
> > -		rc = cmpxchg(&sigp_ctrl->value, old.value, 0);
> > +		WRITE_ONCE(sigp_ctrl->value, 9);
> 
> that's supposed to be a 0, right?

Duh... yes, of course. I added the "9" to better find the corresponding
code in assembly, and obviously forgot to replace it with 0 again.
Thanks for pointing this out!

Strange enough this still worked. Hmm.

Re: [PATCH 2/3] KVM: s390: Remove one byte cmpxchg() usage

Posted by Claudio Imbrenda 1 year, 2 months ago

On Mon, 25 Nov 2024 14:37:55 +0100
Heiko Carstens <hca@linux.ibm.com> wrote:

> On Mon, Nov 25, 2024 at 01:16:17PM +0100, Claudio Imbrenda wrote:
> > On Mon, 25 Nov 2024 12:50:38 +0100
> > Heiko Carstens <hca@linux.ibm.com> wrote:  
> > > @@ -128,23 +126,16 @@ static void sca_clear_ext_call(struct kvm_vcpu *vcpu)
> > >  		struct esca_block *sca = vcpu->kvm->arch.sca;
> > >  		union esca_sigp_ctrl *sigp_ctrl =
> > >  			&(sca->cpu[vcpu->vcpu_id].sigp_ctrl);
> > > -		union esca_sigp_ctrl old;
> > >  
> > > -		old = READ_ONCE(*sigp_ctrl);
> > > -		expect = old.value;
> > > -		rc = cmpxchg(&sigp_ctrl->value, old.value, 0);
> > > +		WRITE_ONCE(sigp_ctrl->value, 9);  
> > 
> > that's supposed to be a 0, right?  
> 
> Duh... yes, of course. I added the "9" to better find the corresponding
> code in assembly, and obviously forgot to replace it with 0 again.
> Thanks for pointing this out!
> 
> Strange enough this still worked. Hmm.

with that fixed:

Acked-by: Claudio Imbrenda <imbrenda@linux.ibm.com>