[PATCH v2 3/4] virtio-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/virtio-blk.c          | 6 +++++-
 hw/core/machine.c              | 1 +
 hw/virtio/virtio-blk-pci.c     | 9 ++++++++-
 include/hw/virtio/virtio-blk.h | 2 ++
 4 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 9bee514c4e..d3ffaffc93 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -1135,6 +1135,9 @@ static void virtio_blk_device_realize(DeviceState *dev, Error **errp)
         error_setg(errp, "Device needs media, but drive is empty");
         return;
     }
+    if (conf->num_queues == VIRTIO_BLK_AUTO_NUM_QUEUES) {
+        conf->num_queues = 1;
+    }
     if (!conf->num_queues) {
         error_setg(errp, "num-queues property must be larger than 0");
         return;
@@ -1271,7 +1274,8 @@ static Property virtio_blk_properties[] = {
 #endif
     DEFINE_PROP_BIT("request-merging", VirtIOBlock, conf.request_merging, 0,
                     true),
-    DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues, 1),
+    DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues,
+                       VIRTIO_BLK_AUTO_NUM_QUEUES),
     DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size, 128),
     DEFINE_PROP_BOOL("seg-max-adjust", VirtIOBlock, conf.seg_max_adjust, true),
     DEFINE_PROP_LINK("iothread", VirtIOBlock, conf.iothread, TYPE_IOTHREAD,
diff --git a/hw/core/machine.c b/hw/core/machine.c
index d6e2370c77..de6ceaa97f 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -28,6 +28,7 @@
 #include "hw/mem/nvdimm.h"
 
 GlobalProperty hw_compat_4_2[] = {
+    { "virtio-blk-device", "num-queues", "1"},
     { "virtio-blk-device", "x-enable-wce-if-config-wce", "off" },
     { "virtio-blk-device", "seg-max-adjust", "off"},
     { "virtio-scsi-device", "num_queues", "1"},
diff --git a/hw/virtio/virtio-blk-pci.c b/hw/virtio/virtio-blk-pci.c
index d9b69a5af3..7e6d863963 100644
--- a/hw/virtio/virtio-blk-pci.c
+++ b/hw/virtio/virtio-blk-pci.c
@@ -17,6 +17,7 @@
 
 #include "qemu/osdep.h"
 
+#include "hw/boards.h"
 #include "hw/qdev-properties.h"
 #include "hw/virtio/virtio-blk.h"
 #include "virtio-pci.h"
@@ -50,9 +51,15 @@ static void virtio_blk_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
 {
     VirtIOBlkPCI *dev = VIRTIO_BLK_PCI(vpci_dev);
     DeviceState *vdev = DEVICE(&dev->vdev);
+    VirtIOBlkConf *conf = &dev->vdev.conf;
+
+    /* 1:1 vq to vcpu mapping is ideal because it avoids IPIs */
+    if (conf->num_queues == VIRTIO_BLK_AUTO_NUM_QUEUES) {
+        conf->num_queues = current_machine->smp.cpus;
+    }
 
     if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
-        vpci_dev->nvectors = dev->vdev.conf.num_queues + 1;
+        vpci_dev->nvectors = conf->num_queues + 1;
     }
 
     qdev_set_parent_bus(vdev, BUS(&vpci_dev->bus));
diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
index 1e62f869b2..4e5e903f4a 100644
--- a/include/hw/virtio/virtio-blk.h
+++ b/include/hw/virtio/virtio-blk.h
@@ -30,6 +30,8 @@ struct virtio_blk_inhdr
     unsigned char status;
 };
 
+#define VIRTIO_BLK_AUTO_NUM_QUEUES UINT16_MAX
+
 struct VirtIOBlkConf
 {
     BlockConf conf;
-- 
2.24.1