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

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Message-ID: <20241125131617.13be742d@p-imbrenda>
Date: Mon, 25 Nov 2024 13:16:17 +0100
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank
 <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] KVM: s390: Remove one byte cmpxchg() usage

On Mon, 25 Nov 2024 12:50:38 +0100
Heiko Carstens <hca@...ux.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@...ux.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)