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

Heiko Carstens
2024-11-25T11:50:49Z

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