From: John Berg <jhnberg@amazon.com>

The MQES field in the CAP register describes the Maximum Queue Entries
Supported for the IO queues of an NVMe controller. Adding a +1 to the
value in this field results in the total queue size. A full queue is
when a queue of size N contains N - 1 entries, and the minimum queue
size is 2. Thus the lowest MQES value is 1.

This patch adds the new mqes property to the NVMe emulation which allows
a user to specify the maximum queue size by setting this property. This
is useful as it enables testing of NVMe controller where the MQES is
relatively small. The smallest NVMe queue size supported in NVMe is 2
submission and completion entries, which means that the smallest legal
mqes value is 1.

The following example shows how the mqes can be set for a the NVMe
emulation:

-drive id=nvme0,if=none,file=nvme.img,format=raw
-device nvme,drive=nvme0,serial=foo,mqes=1

If the mqes property is not provided then the default mqes will still be
0x7ff (the queue size is 2048 entries).

Signed-off-by: John Berg <jhnberg@amazon.co.uk>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
---
 hw/nvme/ctrl.c | 8 +++++++-
 hw/nvme/nvme.h | 1 +
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/hw/nvme/ctrl.c b/hw/nvme/ctrl.c
index 066389e391b6..fa7ec0e79490 100644
--- a/hw/nvme/ctrl.c
+++ b/hw/nvme/ctrl.c
@@ -7805,6 +7805,11 @@ static bool nvme_check_params(NvmeCtrl *n, Error **errp)
         return false;
     }
 
+    if (params->mqes < 1) {
+        error_setg(errp, "mqes property cannot be less than 1");
+        return false;
+    }
+
     if (n->pmr.dev) {
         if (params->msix_exclusive_bar) {
             error_setg(errp, "not enough BARs available to enable PMR");
@@ -8289,7 +8294,7 @@ static void nvme_init_ctrl(NvmeCtrl *n, PCIDevice *pci_dev)
 
     id->ctratt = cpu_to_le32(ctratt);
 
-    NVME_CAP_SET_MQES(cap, 0x7ff);
+    NVME_CAP_SET_MQES(cap, n->params.mqes);
     NVME_CAP_SET_CQR(cap, 1);
     NVME_CAP_SET_TO(cap, 0xf);
     NVME_CAP_SET_CSS(cap, NVME_CAP_CSS_NVM);
@@ -8459,6 +8464,7 @@ static Property nvme_props[] = {
                       params.sriov_max_vq_per_vf, 0),
     DEFINE_PROP_BOOL("msix-exclusive-bar", NvmeCtrl, params.msix_exclusive_bar,
                      false),
+    DEFINE_PROP_UINT16("mqes", NvmeCtrl, params.mqes, 0x7ff),
     DEFINE_PROP_END_OF_LIST(),
 };
 
diff --git a/hw/nvme/nvme.h b/hw/nvme/nvme.h
index bed8191bd5fd..2e7d31c0ae6d 100644
--- a/hw/nvme/nvme.h
+++ b/hw/nvme/nvme.h
@@ -521,6 +521,7 @@ typedef struct NvmeParams {
     uint32_t num_queues; /* deprecated since 5.1 */
     uint32_t max_ioqpairs;
     uint16_t msix_qsize;
+    uint16_t mqes;
     uint32_t cmb_size_mb;
     uint8_t  aerl;
     uint32_t aer_max_queued;
-- 
2.44.0