* [PATCH] mshv: Use try_cmpxchg() instead of cmpxchg()
@ 2026-02-18 10:25 Uros Bizjak
2026-02-18 11:00 ` [PATCH RESEND] " Uros Bizjak
0 siblings, 1 reply; 3+ messages in thread
From: Uros Bizjak @ 2026-02-18 10:25 UTC (permalink / raw)
To: linux-hyperv, linux-kernel
Cc: Uros Bizjak, K. Y. Srinivasan, Haiyang Zhang, Wei Liu, Dexuan Cui,
Long Li
Use !try_cmpxchg() instead of cmpxchg (*ptr, old, new) != old.
x86 CMPXCHG instruction returns success in ZF flag, so this
change saves a compare after cmpxchg().
The generated assembly code improves from e.g.:
415: 48 8b 44 24 30 mov 0x30(%rsp),%rax
41a: 48 8b 54 24 38 mov 0x38(%rsp),%rdx
41f: f0 49 0f b1 91 a8 02 lock cmpxchg %rdx,0x2a8(%r9)
426: 00 00
428: 48 3b 44 24 30 cmp 0x30(%rsp),%rax
42d: 0f 84 09 ff ff ff je 33c <...>
to:
415: 48 8b 44 24 30 mov 0x30(%rsp),%rax
41a: 48 8b 54 24 38 mov 0x38(%rsp),%rdx
41f: f0 49 0f b1 91 a8 02 lock cmpxchg %rdx,0x2a8(%r9)
426: 00 00
428: 0f 84 0e ff ff ff je 33c <...>
No functional change intended.
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Long Li <longli@microsoft.com>
---
drivers/hv/hyperv_vmbus.h | 4 ++--
drivers/hv/mshv_eventfd.c | 4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
index cdbc5f5c3215..7bd8f8486e85 100644
--- a/drivers/hv/hyperv_vmbus.h
+++ b/drivers/hv/hyperv_vmbus.h
@@ -370,8 +370,8 @@ static inline void vmbus_signal_eom(struct hv_message *msg, u32 old_msg_type)
* CHANNELMSG_UNLOAD_RESPONSE and we don't care about other messages
* on crash.
*/
- if (cmpxchg(&msg->header.message_type, old_msg_type,
- HVMSG_NONE) != old_msg_type)
+ if (!try_cmpxchg(&msg->header.message_type,
+ &old_msg_type, HVMSG_NONE))
return;
/*
diff --git a/drivers/hv/mshv_eventfd.c b/drivers/hv/mshv_eventfd.c
index 0b75ff1edb73..525e002758e4 100644
--- a/drivers/hv/mshv_eventfd.c
+++ b/drivers/hv/mshv_eventfd.c
@@ -128,8 +128,8 @@ static int mshv_vp_irq_try_set_vector(struct mshv_vp *vp, u32 vector)
new_iv.vector[new_iv.vector_count++] = vector;
- if (cmpxchg(&vp->vp_register_page->interrupt_vectors.as_uint64,
- iv.as_uint64, new_iv.as_uint64) != iv.as_uint64)
+ if (!try_cmpxchg(&vp->vp_register_page->interrupt_vectors.as_uint64,
+ &iv.as_uint64, new_iv.as_uint64))
return -EAGAIN;
return 0;
--
2.53.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* [PATCH RESEND] mshv: Use try_cmpxchg() instead of cmpxchg()
2026-02-18 10:25 [PATCH] mshv: Use try_cmpxchg() instead of cmpxchg() Uros Bizjak
@ 2026-02-18 11:00 ` Uros Bizjak
2026-02-18 23:27 ` Wei Liu
0 siblings, 1 reply; 3+ messages in thread
From: Uros Bizjak @ 2026-02-18 11:00 UTC (permalink / raw)
To: linux-hyperv, linux-kernel
Cc: Uros Bizjak, K. Y. Srinivasan, Haiyang Zhang, Wei Liu, Dexuan Cui,
Long Li
Use !try_cmpxchg() instead of cmpxchg (*ptr, old, new) != old.
x86 CMPXCHG instruction returns success in ZF flag, so this
change saves a compare after CMPXCHG.
The generated assembly code improves from e.g.:
415: 48 8b 44 24 30 mov 0x30(%rsp),%rax
41a: 48 8b 54 24 38 mov 0x38(%rsp),%rdx
41f: f0 49 0f b1 91 a8 02 lock cmpxchg %rdx,0x2a8(%r9)
426: 00 00
428: 48 3b 44 24 30 cmp 0x30(%rsp),%rax
42d: 0f 84 09 ff ff ff je 33c <...>
to:
415: 48 8b 44 24 30 mov 0x30(%rsp),%rax
41a: 48 8b 54 24 38 mov 0x38(%rsp),%rdx
41f: f0 49 0f b1 91 a8 02 lock cmpxchg %rdx,0x2a8(%r9)
426: 00 00
428: 0f 84 0e ff ff ff je 33c <...>
No functional change intended.
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Long Li <longli@microsoft.com>
---
drivers/hv/hyperv_vmbus.h | 4 ++--
drivers/hv/mshv_eventfd.c | 4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
index cdbc5f5c3215..7bd8f8486e85 100644
--- a/drivers/hv/hyperv_vmbus.h
+++ b/drivers/hv/hyperv_vmbus.h
@@ -370,8 +370,8 @@ static inline void vmbus_signal_eom(struct hv_message *msg, u32 old_msg_type)
* CHANNELMSG_UNLOAD_RESPONSE and we don't care about other messages
* on crash.
*/
- if (cmpxchg(&msg->header.message_type, old_msg_type,
- HVMSG_NONE) != old_msg_type)
+ if (!try_cmpxchg(&msg->header.message_type,
+ &old_msg_type, HVMSG_NONE))
return;
/*
diff --git a/drivers/hv/mshv_eventfd.c b/drivers/hv/mshv_eventfd.c
index 0b75ff1edb73..525e002758e4 100644
--- a/drivers/hv/mshv_eventfd.c
+++ b/drivers/hv/mshv_eventfd.c
@@ -128,8 +128,8 @@ static int mshv_vp_irq_try_set_vector(struct mshv_vp *vp, u32 vector)
new_iv.vector[new_iv.vector_count++] = vector;
- if (cmpxchg(&vp->vp_register_page->interrupt_vectors.as_uint64,
- iv.as_uint64, new_iv.as_uint64) != iv.as_uint64)
+ if (!try_cmpxchg(&vp->vp_register_page->interrupt_vectors.as_uint64,
+ &iv.as_uint64, new_iv.as_uint64))
return -EAGAIN;
return 0;
--
2.53.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH RESEND] mshv: Use try_cmpxchg() instead of cmpxchg()
2026-02-18 11:00 ` [PATCH RESEND] " Uros Bizjak
@ 2026-02-18 23:27 ` Wei Liu
0 siblings, 0 replies; 3+ messages in thread
From: Wei Liu @ 2026-02-18 23:27 UTC (permalink / raw)
To: Uros Bizjak
Cc: linux-hyperv, linux-kernel, K. Y. Srinivasan, Haiyang Zhang,
Wei Liu, Dexuan Cui, Long Li
On Wed, Feb 18, 2026 at 12:00:18PM +0100, Uros Bizjak wrote:
> Use !try_cmpxchg() instead of cmpxchg (*ptr, old, new) != old.
> x86 CMPXCHG instruction returns success in ZF flag, so this
> change saves a compare after CMPXCHG.
>
> The generated assembly code improves from e.g.:
>
> 415: 48 8b 44 24 30 mov 0x30(%rsp),%rax
> 41a: 48 8b 54 24 38 mov 0x38(%rsp),%rdx
> 41f: f0 49 0f b1 91 a8 02 lock cmpxchg %rdx,0x2a8(%r9)
> 426: 00 00
> 428: 48 3b 44 24 30 cmp 0x30(%rsp),%rax
> 42d: 0f 84 09 ff ff ff je 33c <...>
>
> to:
>
> 415: 48 8b 44 24 30 mov 0x30(%rsp),%rax
> 41a: 48 8b 54 24 38 mov 0x38(%rsp),%rdx
> 41f: f0 49 0f b1 91 a8 02 lock cmpxchg %rdx,0x2a8(%r9)
> 426: 00 00
> 428: 0f 84 0e ff ff ff je 33c <...>
>
> No functional change intended.
>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> Cc: "K. Y. Srinivasan" <kys@microsoft.com>
> Cc: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: Wei Liu <wei.liu@kernel.org>
> Cc: Dexuan Cui <decui@microsoft.com>
> Cc: Long Li <longli@microsoft.com>
Applied. Thanks.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2026-02-18 10:25 [PATCH] mshv: Use try_cmpxchg() instead of cmpxchg() Uros Bizjak
2026-02-18 11:00 ` [PATCH RESEND] " Uros Bizjak
2026-02-18 23:27 ` Wei Liu
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