hw/nvme: fix handling of over-committed queues

If a host chooses to use the SQHD "hint" in the CQE to know if there is
room in the submission queue for additional commands, it may result in a
situation where there are not enough internal resources (struct
NvmeRequest) available to process the command. For a lack of a better
term, the host may "over-commit" the device (i.e., it may have more
inflight commands than the queue size).

For example, assume a queue with N entries. The host submits N commands
and all are picked up for processing, advancing the head and emptying
the queue. Regardless of which of these N commands complete first, the
SQHD field of that CQE will indicate to the host that the queue is
empty, which allows the host to issue N commands again. However, if the
device has not posted CQEs for all the previous commands yet, the device
will have less than N resources available to process the commands, so
queue processing is suspended.

And here lies an 11 year latent bug. In the absense of any additional
tail updates on the submission queue, we never schedule the processing
bottom-half again unless we observe a head update on an associated full
completion queue. This has been sufficient to handle N-to-1 SQ/CQ setups
(in the absense of over-commit of course). Incidentially, that "kick all
associated SQs" mechanism can now be killed since we now just schedule
queue processing when we return a processing resource to a non-empty
submission queue, which happens to cover both edge cases. However, we
must retain kicking the CQ if it was previously full.

So, apparently, no previous driver tested with hw/nvme has ever used
SQHD (e.g., neither the Linux NVMe driver or SPDK uses it). But then OSv
shows up with the driver that actually does. I salute you.

Fixes: f3c507a ("NVMe: Initial commit for new storage interface")
Cc: [email protected]
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2388
Reported-by: Waldemar Kozaczuk <[email protected]>
Reviewed-by: Keith Busch <[email protected]>
Signed-off-by: Klaus Jensen <[email protected]>
(cherry picked from commit 9529aa6)
Signed-off-by: Michael Tokarev <[email protected]>

hw/nvme/ctrl.c

@@ -1516,9 +1516,16 @@ static void nvme_post_cqes(void *opaque)
             stl_le_p(&n->bar.csts, NVME_CSTS_FAILED);
             break;
         }
+
         QTAILQ_REMOVE(&cq->req_list, req, entry);
+
         nvme_inc_cq_tail(cq);
         nvme_sg_unmap(&req->sg);
+
+        if (QTAILQ_EMPTY(&sq->req_list) && !nvme_sq_empty(sq)) {
+            qemu_bh_schedule(sq->bh);
+        }
+
         QTAILQ_INSERT_TAIL(&sq->req_list, req, entry);
     }
     if (cq->tail != cq->head) {
@@ -7806,7 +7813,6 @@ static void nvme_process_db(NvmeCtrl *n, hwaddr addr, int val)
         /* Completion queue doorbell write */
 
         uint16_t new_head = val & 0xffff;
-        int start_sqs;
         NvmeCQueue *cq;
 
         qid = (addr - (0x1000 + (1 << 2))) >> 3;
@@ -7857,18 +7863,15 @@ static void nvme_process_db(NvmeCtrl *n, hwaddr addr, int val)
 
         trace_pci_nvme_mmio_doorbell_cq(cq->cqid, new_head);
 
-        start_sqs = nvme_cq_full(cq) ? 1 : 0;
+        /* scheduled deferred cqe posting if queue was previously full */
+        if (nvme_cq_full(cq)) {
+            qemu_bh_schedule(cq->bh);
+        }
+
         cq->head = new_head;
         if (!qid && n->dbbuf_enabled) {
             stl_le_pci_dma(pci, cq->db_addr, cq->head, MEMTXATTRS_UNSPECIFIED);
         }
-        if (start_sqs) {
-            NvmeSQueue *sq;
-            QTAILQ_FOREACH(sq, &cq->sq_list, entry) {
-                qemu_bh_schedule(sq->bh);
-            }
-            qemu_bh_schedule(cq->bh);
-        }
 
         if (cq->tail == cq->head) {
             if (cq->irq_enabled) {