* [PATCH] KVM: s390: interrupt: use READ_ONCE() before cmpxchg()
@ 2023-01-09 12:51 Heiko Carstens
2023-01-09 14:50 ` Heiko Carstens
0 siblings, 1 reply; 2+ messages in thread
From: Heiko Carstens @ 2023-01-09 12:51 UTC (permalink / raw)
To: Christian Borntraeger, Janosch Frank, Claudio Imbrenda
Cc: David Hildenbrand, kvm, linux-s390, linux-kernel
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
^ permalink raw reply related [flat|nested] 2+ messages in thread* Re: [PATCH] KVM: s390: interrupt: use READ_ONCE() before cmpxchg()
2023-01-09 12:51 [PATCH] KVM: s390: interrupt: use READ_ONCE() before cmpxchg() Heiko Carstens
@ 2023-01-09 14:50 ` Heiko Carstens
0 siblings, 0 replies; 2+ messages in thread
From: Heiko Carstens @ 2023-01-09 14:50 UTC (permalink / raw)
To: Heiko Carstens
Cc: Christian Borntraeger, Janosch Frank, Claudio Imbrenda,
David Hildenbrand, kvm, linux-s390, linux-kernel
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!
^ permalink raw reply [flat|nested] 2+ messages in thread
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