Re: [PATCH 1/2] block/export: add vhost-user-blk multi-queue support

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Oct 02, 2020 at 07:32:39AM +0200, Markus Armbruster wrote:
> Stefan Hajnoczi <stefanha@redhat.com> writes:
> 
> > Allow the number of queues to be configured using --export
> > vhost-user-blk,num-queues=N. This setting should match the QEMU --device
> > vhost-user-blk-pci,num-queues=N setting but QEMU vhost-user-blk.c lowers
> > its own value if the vhost-user-blk backend offers fewer queues than
> > QEMU.
> >
> > The vhost-user-blk-server.c code is already capable of multi-queue. All
> > virtqueue processing runs in the same AioContext. No new locking is
> > needed.
> >
> > Add the num-queues=N option and set the VIRTIO_BLK_F_MQ feature bit.
> > Note that the feature bit only announces the presence of the num_queues
> > configuration space field. It does not promise that there is more than 1
> > virtqueue, so we can set it unconditionally.
> >
> > I tested multi-queue by running a random read fio test with numjobs=4 on
> > an -smp 4 guest. After the benchmark finished the guest /proc/interrupts
> > file showed activity on all 4 virtio-blk MSI-X. The /sys/block/vda/mq/
> > directory shows that Linux blk-mq has 4 queues configured.
> >
> > An automated test is included in the next commit.
> >
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  qapi/block-export.json               |  6 +++++-
> >  block/export/vhost-user-blk-server.c | 24 ++++++++++++++++++------
> >  2 files changed, 23 insertions(+), 7 deletions(-)
> >
> > diff --git a/qapi/block-export.json b/qapi/block-export.json
> > index a793e34af9..17020de257 100644
> > --- a/qapi/block-export.json
> > +++ b/qapi/block-export.json
> > @@ -93,11 +93,15 @@
> >  #        SocketAddress types are supported. Passed fds must be UNIX domain
> >  #        sockets.
> >  # @logical-block-size: Logical block size in bytes. Defaults to 512 bytes.
> > +# @num-queues: Number of request virtqueues. Must be greater than 0. Defaults
> > +#              to 1.
> >  #
> >  # Since: 5.2
> >  ##
> >  { 'struct': 'BlockExportOptionsVhostUserBlk',
> > -  'data': { 'addr': 'SocketAddress', '*logical-block-size': 'size' } }
> > +  'data': { 'addr': 'SocketAddress',
> > +	    '*logical-block-size': 'size',
> 
> Tab damage.

Oops, thanks! I have updated my editor configuration to use 4-space
indents for .json files :).

> > +            '*num-queues': 'uint16'} }
> 
> Out of curiosity: what made you pick 16 bit signed?  net.json uses both
> 32 and 64 bit signed.  Odd...

struct virtio_blk_config {
    __u16 num_queues;
}

Also, virtio-pci and virtio-ccw use 16-bit types for the queue count.

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