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

Claudio Imbrenda
2024-11-25T12:16:28Z

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)