[PATCH v2 4/4] vhost-user-blk: default num_queues to -smp N

[Stefan Hajnoczi <stefanha@redhat.com>]

Automatically size the number of request virtqueues to match the number
of vCPUs.  This ensures that completion interrupts are handled on the
same vCPU that submitted the request.  No IPI is necessary to complete
an I/O request and performance is improved.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/block/vhost-user-blk.c          | 6 +++++-
 hw/core/machine.c                  | 1 +
 hw/virtio/vhost-user-blk-pci.c     | 6 ++++++
 include/hw/virtio/vhost-user-blk.h | 2 ++
 4 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index 98b383f90e..2ee26a434c 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -403,6 +403,9 @@ static void vhost_user_blk_device_realize(DeviceState *dev, Error **errp)
         return;
     }
 
+    if (s->num_queues == VHOST_USER_BLK_AUTO_NUM_QUEUES) {
+        s->num_queues = 1;
+    }
     if (!s->num_queues || s->num_queues > VIRTIO_QUEUE_MAX) {
         error_setg(errp, "vhost-user-blk: invalid number of IO queues");
         return;
@@ -500,7 +503,8 @@ static const VMStateDescription vmstate_vhost_user_blk = {
 
 static Property vhost_user_blk_properties[] = {
     DEFINE_PROP_CHR("chardev", VHostUserBlk, chardev),
-    DEFINE_PROP_UINT16("num-queues", VHostUserBlk, num_queues, 1),
+    DEFINE_PROP_UINT16("num-queues", VHostUserBlk, num_queues,
+                       VHOST_USER_BLK_AUTO_NUM_QUEUES),
     DEFINE_PROP_UINT32("queue-size", VHostUserBlk, queue_size, 128),
     DEFINE_PROP_BIT("config-wce", VHostUserBlk, config_wce, 0, true),
     DEFINE_PROP_END_OF_LIST(),
diff --git a/hw/core/machine.c b/hw/core/machine.c
index de6ceaa97f..d4c67f4d6e 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -35,6 +35,7 @@ GlobalProperty hw_compat_4_2[] = {
     { "virtio-scsi-device", "seg_max_adjust", "off"},
     { "vhost-blk-device", "seg_max_adjust", "off"},
     { "vhost-scsi", "num_queues", "1"},
+    { "vhost-user-blk", "num-queues", "1"},
     { "vhost-user-scsi", "num_queues", "1"},
     { "usb-host", "suppress-remote-wake", "off" },
     { "usb-redir", "suppress-remote-wake", "off" },
diff --git a/hw/virtio/vhost-user-blk-pci.c b/hw/virtio/vhost-user-blk-pci.c
index 1dc834a3ff..cf72b21c16 100644
--- a/hw/virtio/vhost-user-blk-pci.c
+++ b/hw/virtio/vhost-user-blk-pci.c
@@ -19,6 +19,7 @@
 #include "qemu/osdep.h"
 
 #include "standard-headers/linux/virtio_pci.h"
+#include "hw/boards.h"
 #include "hw/virtio/virtio.h"
 #include "hw/virtio/vhost-user-blk.h"
 #include "hw/pci/pci.h"
@@ -54,6 +55,11 @@ static void vhost_user_blk_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
     VHostUserBlkPCI *dev = VHOST_USER_BLK_PCI(vpci_dev);
     DeviceState *vdev = DEVICE(&dev->vdev);
 
+    /* 1:1 vq to vcpu mapping is ideal because it avoids IPIs */
+    if (dev->vdev.num_queues == VHOST_USER_BLK_AUTO_NUM_QUEUES) {
+        dev->vdev.num_queues = current_machine->smp.cpus;
+    }
+
     if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
         vpci_dev->nvectors = dev->vdev.num_queues + 1;
     }
diff --git a/include/hw/virtio/vhost-user-blk.h b/include/hw/virtio/vhost-user-blk.h
index 108bfadeeb..5a353dc1c6 100644
--- a/include/hw/virtio/vhost-user-blk.h
+++ b/include/hw/virtio/vhost-user-blk.h
@@ -25,6 +25,8 @@
 #define VHOST_USER_BLK(obj) \
         OBJECT_CHECK(VHostUserBlk, (obj), TYPE_VHOST_USER_BLK)
 
+#define VHOST_USER_BLK_AUTO_NUM_QUEUES UINT16_MAX
+
 typedef struct VHostUserBlk {
     VirtIODevice parent_obj;
     CharBackend chardev;
-- 
2.24.1