Re: [Qemu-devel] [PATCH -qemu] nvme: support Google vendor extension

[Ming Lin <mlin@kernel.org>]

On Thu, 2015-11-19 at 11:37 +0100, Paolo Bonzini wrote:
> 
> On 18/11/2015 06:47, Ming Lin wrote:
> > @@ -726,7 +798,11 @@ static void nvme_process_db(NvmeCtrl *n, hwaddr addr, int val)
> >          }
> >  
> >          start_sqs = nvme_cq_full(cq) ? 1 : 0;
> > -        cq->head = new_head;
> > +        /* When the mapped pointer memory area is setup, we don't rely on
> > +         * the MMIO written values to update the head pointer. */
> > +        if (!cq->db_addr) {
> > +            cq->head = new_head;
> > +        }
> 
> You are still checking
> 
>         if (new_head >= cq->size) {
>             return;
>         }
> 
> above.  I think this is incorrect when the extension is present, and
> furthermore it's the only case where val is being used.
> 
> If you're not using val, you could use ioeventfd for the MMIO.  An
> ioeventfd cuts the MMIO cost by at least 55% and up to 70%. Here are
> quick and dirty measurements from kvm-unit-tests's vmexit.flat
> benchmark, on two very different machines:
> 
> 			Haswell-EP		Ivy Bridge i7
>   MMIO memory write	5100 -> 2250 (55%)	7000 -> 3000 (58%)
>   I/O port write	3800 -> 1150 (70%)	4100 -> 1800 (57%)
> 
> You would need to allocate two eventfds for each qid, one for the sq and
> one for the cq.  Also, processing the queues is now bounced to the QEMU
> iothread, so you can probably get rid of sq->timer and cq->timer.

Here is a quick try.
Too late now, I'll test it morning.

Do you see obvious problem?

diff --git a/hw/block/nvme.c b/hw/block/nvme.c
index 3e1c38d..d28690d 100644
--- a/hw/block/nvme.c
+++ b/hw/block/nvme.c
@@ -543,6 +543,44 @@ static uint16_t nvme_set_feature(NvmeCtrl *n, NvmeCmd *cmd, NvmeRequest *req)
     return NVME_SUCCESS;
 }
 
+static void nvme_cq_notifier(EventNotifier *e)
+{
+    NvmeCQueue *cq =
+        container_of(e, NvmeCQueue, notifier);
+
+    nvme_post_cqes(cq);
+}
+
+static void nvme_init_cq_eventfd(NvmeCQueue *cq)
+{
+    NvmeCtrl *n = cq->ctrl;
+    uint16_t offset = (cq->cqid*2+1) * NVME_CAP_DSTRD(n->bar.cap);
+
+    event_notifier_init(&cq->notifier, 0);
+    event_notifier_set_handler(&cq->notifier, nvme_cq_notifier);
+    memory_region_add_eventfd(&n->iomem,
+        0x1000 + offset, 4, true, cq->cqid*2+1, &cq->notifier);
+}
+
+static void nvme_sq_notifier(EventNotifier *e)
+{
+    NvmeSQueue *sq =
+        container_of(e, NvmeSQueue, notifier);
+
+    nvme_process_sq(sq);
+}
+
+static void nvme_init_sq_eventfd(NvmeSQueue *sq)
+{
+    NvmeCtrl *n = sq->ctrl;
+    uint16_t offset = sq->sqid * 2 * NVME_CAP_DSTRD(n->bar.cap);
+
+    event_notifier_init(&sq->notifier, 0);
+    event_notifier_set_handler(&sq->notifier, nvme_sq_notifier);
+    memory_region_add_eventfd(&n->iomem,
+        0x1000 + offset, 4, true, sq->sqid * 2, &sq->notifier);
+}
+
 static uint16_t nvme_set_db_memory(NvmeCtrl *n, const NvmeCmd *cmd)
 {
     uint64_t db_addr = le64_to_cpu(cmd->prp1);
@@ -565,6 +603,7 @@ static uint16_t nvme_set_db_memory(NvmeCtrl *n, const NvmeCmd *cmd)
             /* Submission queue tail pointer location, 2 * QID * stride. */
             sq->db_addr = db_addr + 2 * i * 4;
             sq->eventidx_addr = eventidx_addr + 2 * i * 4;
+            nvme_init_sq_eventfd(sq);
         }
 
         if (cq != NULL) {
@@ -572,6 +611,7 @@ static uint16_t nvme_set_db_memory(NvmeCtrl *n, const NvmeCmd *cmd)
              */
             cq->db_addr = db_addr + (2 * i + 1) * 4;
             cq->eventidx_addr = eventidx_addr + (2 * i + 1) * 4;
+            nvme_init_cq_eventfd(cq);
         }
     }
     return NVME_SUCCESS;
@@ -793,7 +833,7 @@ static void nvme_process_db(NvmeCtrl *n, hwaddr addr, int val)
         }
 
         cq = n->cq[qid];
-        if (new_head >= cq->size) {
+        if (!cq->db_addr && new_head >= cq->size) {
             return;
         }
 
diff --git a/hw/block/nvme.h b/hw/block/nvme.h
index 82aeab4..608f202 100644
--- a/hw/block/nvme.h
+++ b/hw/block/nvme.h
@@ -667,6 +667,7 @@ typedef struct NvmeSQueue {
      * do not go over this value will not result in MMIO writes (but will
      * still write the tail pointer to the "db_addr" location above). */
     uint64_t    eventidx_addr;
+    EventNotifier notifier;
 } NvmeSQueue;
 
 typedef struct NvmeCQueue {
@@ -689,6 +690,7 @@ typedef struct NvmeCQueue {
      * do not go over this value will not result in MMIO writes (but will
      * still write the head pointer to the "db_addr" location above). */
     uint64_t    eventidx_addr;
+    EventNotifier notifier;
 } NvmeCQueue;
 
 typedef struct NvmeNamespace {

> 
> Paolo