2023-01-09 15:48:27

by Heiko Carstens

Subject: [PATCH v2] 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.

Signed-off-by: Heiko Carstens <[email protected]>
---

v2: make it compile

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..ab26aa53ee37 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;

+ 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);
}
--
2.34.1

2023-01-09 16:31:35

by Claudio Imbrenda

Subject: Re: [PATCH v2] KVM: s390: interrupt: use READ_ONCE() before cmpxchg()

On Mon, 9 Jan 2023 15:54:56 +0100
Heiko Carstens <[email protected]> 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 <[email protected]>

Reviewed-by: Claudio Imbrenda <[email protected]>

> ---
>
> v2: make it compile
>
> 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..ab26aa53ee37 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;
>
> + 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);
> }

2023-01-10 19:37:35

by Christian Borntraeger

Subject: Re: [PATCH v2] KVM: s390: interrupt: use READ_ONCE() before cmpxchg()

Am 09.01.23 um 15:54 schrieb Heiko Carstens:
> 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 <[email protected]>

Looks sane.

Reviewed-by: Christian Borntraeger <[email protected]>

In case you want to take it via the s390 tree:
Acked-by: Christian Borntraeger <[email protected]>

> ---
>
> v2: make it compile
>
> 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..ab26aa53ee37 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;
>
> + 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);
> }

2023-01-11 13:05:22

by Heiko Carstens

Subject: Re: [PATCH v2] KVM: s390: interrupt: use READ_ONCE() before cmpxchg()

On Tue, Jan 10, 2023 at 08:15:50PM +0100, Christian Borntraeger wrote:
> Am 09.01.23 um 15:54 schrieb Heiko Carstens:
> > 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 <[email protected]>
>
> Looks sane.
>
> Reviewed-by: Christian Borntraeger <[email protected]>
>
> In case you want to take it via the s390 tree:
> Acked-by: Christian Borntraeger <[email protected]>

Given that there don't seem to be any pending s390 specific patches I
take it via the s390 tree.

Applied.