[v1] mshv: Use try_cmpxchg() instead of cmpxchg()

[PATCH] mshv: Use try_cmpxchg() instead of cmpxchg()

Posted by Uros Bizjak 1 month, 1 week ago

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

[PATCH RESEND] mshv: Use try_cmpxchg() instead of cmpxchg()

Posted by Uros Bizjak 1 month, 1 week ago

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

Re: [PATCH RESEND] mshv: Use try_cmpxchg() instead of cmpxchg()

Posted by Wei Liu 1 month, 1 week ago

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.