KVM: s390: interrupt: use READ_ONCE() before cmpxchg()

Use READ_ONCE() before cmpxchg() to prevent that the compiler generates
code that fetches the to be compared old value several times from memory.

Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Link: https://lore.kernel.org/r/20230109145456.2895385-1-hca@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index 1dae78deddf28602d32d051dc5ad956be6dd5315..ab26aa53ee3713bbc54bb4f4d08e9b6689c8de34 100644 (file)
--- 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);
        }