[PATCH v7 RFC] nvme: improve performance for virtual NVMe devices

[Helen Koike]

Hi Christoph,

Thanks for reviewing this patch

On 2017-03-31 04:01 AM, Christoph Hellwig wrote:
> I'd still prefer to not hide the MMIO path behind nvme_write_doorbell,
> but except for that this looks fine to me:
>

Do you mean wrapping the call to writel()? What do you think if I send a new
version with the changes below? It wouldn't hide the call to writel()
inside nvme_write_doorbell anymore.

Helen

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index 3ce705a..0e5f2a1 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -277,8 +277,9 @@ static inline int nvme_dbbuf_need_event(u16 
event_idx, u16 new_idx, u16 old)
         return (u16)(new_idx - event_idx - 1) < (u16)(new_idx - old);
  }

-static void nvme_write_doorbell(u16 value, u32 __iomem *db, u32 *dbbuf_db,
-                               volatile u32 *dbbuf_ei)
+/* Update dbbuf and return true if a MMIO is required */
+static bool nvme_dbbuf_update_and_check_event(u16 value, u32 *dbbuf_db,
+                                             volatile u32 *dbbuf_ei)
  {
         if (dbbuf_db) {
                 u16 old_value;
@@ -291,11 +292,12 @@ static void nvme_write_doorbell(u16 value, u32 
__iomem *db, u32 *dbbuf_db,

                 old_value = *dbbuf_db;
                 *dbbuf_db = value;
+
                 if (!nvme_dbbuf_need_event(*dbbuf_ei, value, old_value))
-                       return;
+                       return false;
         }

-       writel(value, db);
+       return true;
  }

  /*
@@ -422,8 +424,9 @@ static void __nvme_submit_cmd(struct nvme_queue *nvmeq,

         if (++tail == nvmeq->q_depth)
                 tail = 0;
-       nvme_write_doorbell(tail, nvmeq->q_db, nvmeq->dbbuf_sq_db,
-                           nvmeq->dbbuf_sq_ei);
+       if (nvme_dbbuf_update_and_check_event(tail, nvmeq->dbbuf_sq_db,
+                                             nvmeq->dbbuf_sq_ei))
+               writel(tail, nvmeq->q_db);
         nvmeq->sq_tail = tail;
  }

@@ -839,8 +842,9 @@ static void __nvme_process_cq(struct nvme_queue 
*nvmeq, unsigned int *tag)
                 return;

         if (likely(nvmeq->cq_vector >= 0))
-               nvme_write_doorbell(head, nvmeq->q_db + 
nvmeq->dev->db_stride,
-                                   nvmeq->dbbuf_cq_db, nvmeq->dbbuf_cq_ei);
+               if (nvme_dbbuf_update_and_check_event(head, 
nvmeq->dbbuf_cq_db,
+                                                     nvmeq->dbbuf_cq_ei))
+                       writel(head, nvmeq->q_db + nvmeq->dev->db_stride);
         nvmeq->cq_head = head;
         nvmeq->cq_phase = phase;