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>
---
 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..a3bf1b6a7962 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 = 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 = 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);
 	}
-- 
2.34.1

On Mon, Jan 09, 2023 at 01:51:35PM +0100, Heiko Carstens 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>
> ---
>  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..a3bf1b6a7962 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 = READ_ONCE(*sigp_ctrl);

FWIW, Claudio pointed out that this doesn't even compile.
Sending a new version - sorry about this!