From 9529aa6bb4d18763f5b4704cb4198bd25cbbee31 Mon Sep 17 00:00:00 2001 From: Klaus Jensen Date: Tue, 29 Oct 2024 13:15:19 +0100 Subject: [PATCH] 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: f3c507adcd7b ("NVMe: Initial commit for new storage interface") Cc: qemu-stable@nongnu.org Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2388 Reported-by: Waldemar Kozaczuk Reviewed-by: Keith Busch Signed-off-by: Klaus Jensen --- hw/nvme/ctrl.c | 21 ++++++++++++--------- 1 file changed, 12 insertions(+), 9 deletions(-) diff --git a/hw/nvme/ctrl.c b/hw/nvme/ctrl.c index 8e4612e035..69bce20f66 100644 --- a/hw/nvme/ctrl.c +++ b/hw/nvme/ctrl.c @@ -1520,9 +1520,16 @@ 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) { @@ -7981,7 +7988,6 @@ 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; @@ -8032,18 +8038,15 @@ 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) {