mshv: Use try_cmpxchg() instead of cmpxchg()

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>
Signed-off-by: Wei Liu <wei.liu@kernel.org>

diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
index cdbc5f5..7bd8f84 100644 (file)
--- 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 5e0b10a..492c625 100644 (file)
--- a/drivers/hv/mshv_eventfd.c
+++ b/drivers/hv/mshv_eventfd.c
@@ -129,8 +129,8 @@ 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;