[PATCH v2 2/3] KVM: s390: Remove one byte cmpxchg() usage

[Heiko Carstens <hca@linux.ibm.com>]

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().

Acked-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
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..ea8dce299954 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, 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;
 
-		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