From: Alexey Dobriyan <adobriyan@gmail.com>

Date: Fri, 28 Feb 2020 21:45:19 +0300

Subject: [PATCH] nvme-pci: slimmer CQ head update

Git-commit: e2a366a4b0feaeba8f0bf6091ddd2ac27507a9d3

Patch-mainline: v5.7-rc1

References: git-fixes

Update CQ head with pre-increment operator. This saves subtraction of 1

and a few registers.

Also update phase with "^= 1". This generates only one RMW instruction.

	ffffffff815ba150 <nvme_update_cq_head>:

	ffffffff815ba150:       0f b7 47 70             movzx  eax,WORD PTR [rdi+0x70]

	ffffffff815ba154:       83 c0 01                add    eax,0x1

	ffffffff815ba157:       66 89 47 70             mov    WORD PTR [rdi+0x70],ax

	ffffffff815ba15b:       66 3b 47 68             cmp    ax,WORD PTR [rdi+0x68]

	ffffffff815ba15f:       74 01                   je     ffffffff815ba162 <nvme_update_cq_head+0x12>

	ffffffff815ba161:       c3                      ret

	ffffffff815ba162:       31 c0                   xor    eax,eax

	ffffffff815ba164:       80 77 74 01      ===>   xor    BYTE PTR [rdi+0x74],0x1

	ffffffff815ba168:       66 89 47 70             mov    WORD PTR [rdi+0x70],ax

	ffffffff815ba16c:       c3                      ret

	add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-119 (-119)

	Function                                     old     new   delta

	nvme_poll                                    690     678     -12

	nvme_dev_disable                            1230    1177     -53

	nvme_irq                                     613     559     -54

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>

Acked-by: Hannes Reinecke <hare@suse.com>

---

 drivers/nvme/host/pci.c | 6 ++----

 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c

index d3f23d6254e4..cdc9b6149d38 100644

--- a/drivers/nvme/host/pci.c

+++ b/drivers/nvme/host/pci.c

@@ -982,11 +982,9 @@ static void nvme_complete_cqes(struct nvme_queue *nvmeq, u16 start, u16 end)

 static inline void nvme_update_cq_head(struct nvme_queue *nvmeq)

 {

-	if (nvmeq->cq_head == nvmeq->q_depth - 1) {

+	if (++nvmeq->cq_head == nvmeq->q_depth) {

 		nvmeq->cq_head = 0;

-		nvmeq->cq_phase = !nvmeq->cq_phase;

-	} else {

-		nvmeq->cq_head++;

+		nvmeq->cq_phase ^= 1;

 	}

 }

-- 

2.29.2