Re: [PATCH v2] KVM: s390: interrupt: use READ_ONCE() before cmpxchg()

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Mon,  9 Jan 2023 15:54:56 +0100
Heiko Carstens <hca@linux.ibm.com> wrote:

> Use READ_ONCE() before cmpxchg() to prevent that the compiler generates
> code that fetches the to be compared old value several times from memory.
> 
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

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

> ---
> 
> v2: make it compile
> 
> arch/s390/kvm/interrupt.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
> index 1dae78deddf2..ab26aa53ee37 100644
> --- a/arch/s390/kvm/interrupt.c
> +++ b/arch/s390/kvm/interrupt.c
> @@ -83,8 +83,9 @@ static int sca_inject_ext_call(struct kvm_vcpu *vcpu, int src_id)
>  		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 new_val = {0}, old_val = *sigp_ctrl;
> +		union esca_sigp_ctrl new_val = {0}, old_val;
>  
> +		old_val = READ_ONCE(*sigp_ctrl);
>  		new_val.scn = src_id;
>  		new_val.c = 1;
>  		old_val.c = 0;
> @@ -95,8 +96,9 @@ static int sca_inject_ext_call(struct kvm_vcpu *vcpu, int src_id)
>  		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 new_val = {0}, old_val = *sigp_ctrl;
> +		union bsca_sigp_ctrl new_val = {0}, old_val;
>  
> +		old_val = READ_ONCE(*sigp_ctrl);
>  		new_val.scn = src_id;
>  		new_val.c = 1;
>  		old_val.c = 0;
> @@ -126,16 +128,18 @@ 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 = *sigp_ctrl;
> +		union esca_sigp_ctrl old;
>  
> +		old = READ_ONCE(*sigp_ctrl);
>  		expect = old.value;
>  		rc = cmpxchg(&sigp_ctrl->value, old.value, 0);
>  	} 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 = *sigp_ctrl;
> +		union bsca_sigp_ctrl old;
>  
> +		old = READ_ONCE(*sigp_ctrl);
>  		expect = old.value;
>  		rc = cmpxchg(&sigp_ctrl->value, old.value, 0);
>  	}