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[PATCH 1/1] virtio-blk-ccw: tweak the default for num_queues

From: Halil Pasic
Subject: [PATCH 1/1] virtio-blk-ccw: tweak the default for num_queues
Date: Mon, 9 Nov 2020 16:48:31 +0100

Currently the default value of num_queues is effectively 1 for
virtio-blk-ccw. Recently 9445e1e15e ("virtio-blk-pci: default num_queues
to -smp N") changed the default for pci to the number of vcpus, citing
interrupt better locality and better performance as a rationale.

While virtio-blk-ccw does not yet benefit from better interrupt
locality, measurements have shown that for secure VMs multiqueue does
help with performance. Since the bounce buffering happens with the queue
lock held (in the guest) this is hardly a surprise.

As for non-secure VMs the extra queues shouldn't hurt too much.

Suggested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Halil Pasic <pasic@linux.ibm.com>

We would prefer to land this for 5.2. If we do then commit 9445e1e15e
("virtio-blk-pci: default num_queues to -smp N") took care of all the
necessary compat handling.

If that's not possible, I will send a version that does the necessary
compat handling.
 hw/s390x/virtio-ccw-blk.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/hw/s390x/virtio-ccw-blk.c b/hw/s390x/virtio-ccw-blk.c
index 2294ce1ce4..7296140dde 100644
--- a/hw/s390x/virtio-ccw-blk.c
+++ b/hw/s390x/virtio-ccw-blk.c
@@ -10,6 +10,7 @@
 #include "qemu/osdep.h"
+#include "hw/boards.h"
 #include "hw/qdev-properties.h"
 #include "hw/virtio/virtio.h"
 #include "qapi/error.h"
@@ -20,6 +21,11 @@ static void virtio_ccw_blk_realize(VirtioCcwDevice *ccw_dev, 
Error **errp)
     VirtIOBlkCcw *dev = VIRTIO_BLK_CCW(ccw_dev);
     DeviceState *vdev = DEVICE(&dev->vdev);
+    VirtIOBlkConf *conf = &dev->vdev.conf;
+    if (conf->num_queues == VIRTIO_BLK_AUTO_NUM_QUEUES) {
+        conf->num_queues = MIN(4, current_machine->smp.cpus);
+    }
     qdev_realize(vdev, BUS(&ccw_dev->bus), errp);

base-commit: 2a190a7256a3e0563b29ffd67e0164097b4a6dac

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