[PATCH 4/5] 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          | 17 +++++++++++++++++
 hw/core/machine.c              |  1 +
 hw/virtio/virtio-blk-pci.c     |  3 ++-
 include/hw/virtio/virtio-blk.h |  1 +
 4 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 9bee514c4e..8966b5a571 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -18,6 +18,7 @@
 #include "qemu/error-report.h"
 #include "qemu/main-loop.h"
 #include "trace.h"
+#include "hw/boards.h"
 #include "hw/block/block.h"
 #include "hw/qdev-properties.h"
 #include "sysemu/blockdev.h"
@@ -1119,6 +1120,18 @@ static const BlockDevOps virtio_block_ops = {
     .resize_cb = virtio_blk_resize,
 };
 
+static uint32_t virtio_blk_get_num_virtqueues(VirtIODevice *vdev)
+{
+    VirtIOBlock *s = VIRTIO_BLK(vdev);
+    VirtIOBlkConf *conf = &s->conf;
+
+    if (conf->num_queues == 1 && conf->auto_num_queues) {
+        return current_machine->smp.cpus;
+    }
+
+    return conf->num_queues;
+}
+
 static void virtio_blk_device_realize(DeviceState *dev, Error **errp)
 {
     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
@@ -1135,6 +1148,7 @@ static void virtio_blk_device_realize(DeviceState *dev, Error **errp)
         error_setg(errp, "Device needs media, but drive is empty");
         return;
     }
+    conf->num_queues = virtio_blk_get_num_virtqueues(vdev);
     if (!conf->num_queues) {
         error_setg(errp, "num-queues property must be larger than 0");
         return;
@@ -1272,6 +1286,8 @@ static Property virtio_blk_properties[] = {
     DEFINE_PROP_BIT("request-merging", VirtIOBlock, conf.request_merging, 0,
                     true),
     DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues, 1),
+    DEFINE_PROP_BOOL("auto-num-queues", VirtIOBlock,
+                     conf.auto_num_queues, true),
     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,
@@ -1297,6 +1313,7 @@ static void virtio_blk_class_init(ObjectClass *klass, void *data)
     dc->props = virtio_blk_properties;
     dc->vmsd = &vmstate_virtio_blk;
     set_bit(DEVICE_CATEGORY_STORAGE, dc->categories);
+    vdc->get_num_virtqueues = virtio_blk_get_num_virtqueues;
     vdc->realize = virtio_blk_device_realize;
     vdc->unrealize = virtio_blk_device_unrealize;
     vdc->get_config = virtio_blk_update_config;
diff --git a/hw/core/machine.c b/hw/core/machine.c
index 41da19d85b..a33d1e93b1 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", "auto-num-queues", "off"},
     { "virtio-blk-device", "x-enable-wce-if-config-wce", "off" },
     { "virtio-blk-device", "seg-max-adjust", "off"},
     { "virtio-scsi-device", "auto_num_queues", "off"},
diff --git a/hw/virtio/virtio-blk-pci.c b/hw/virtio/virtio-blk-pci.c
index d9b69a5af3..5131955cec 100644
--- a/hw/virtio/virtio-blk-pci.c
+++ b/hw/virtio/virtio-blk-pci.c
@@ -50,9 +50,10 @@ static void virtio_blk_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
 {
     VirtIOBlkPCI *dev = VIRTIO_BLK_PCI(vpci_dev);
     DeviceState *vdev = DEVICE(&dev->vdev);
+    VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
 
     if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
-        vpci_dev->nvectors = dev->vdev.conf.num_queues + 1;
+        vpci_dev->nvectors = vdc->get_num_virtqueues(VIRTIO_DEVICE(vdev)) + 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..375d7b920a 100644
--- a/include/hw/virtio/virtio-blk.h
+++ b/include/hw/virtio/virtio-blk.h
@@ -37,6 +37,7 @@ struct VirtIOBlkConf
     char *serial;
     uint32_t request_merging;
     uint16_t num_queues;
+    bool auto_num_queues;
     uint16_t queue_size;
     bool seg_max_adjust;
     uint32_t max_discard_sectors;
-- 
2.24.1