Re: [PATCH v4 1/5] virtio-pci: add virtio_pci_optimal_num_queues() helper

[Cornelia Huck]

On Wed, 27 May 2020 11:29:21 +0100
Stefan Hajnoczi <stefanha@redhat.com> wrote:

> Multi-queue devices achieve the best performance when each vCPU has a
> dedicated queue. This ensures that virtqueue used notifications are
> handled on the same vCPU that submitted virtqueue buffers.  When another
> vCPU handles the the notification an IPI will be necessary to wake the
> submission vCPU and this incurs a performance overhead.
> 
> Provide a helper function that virtio-pci devices will use in later
> patches to automatically select the optimal number of queues.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  hw/virtio/virtio-pci.h | 9 +++++++++
>  hw/virtio/virtio-pci.c | 7 +++++++
>  2 files changed, 16 insertions(+)

That looks like a good idea, since the policy can be easily tweaked in
one place later.

For ccw, I don't see a good way to arrive at an optimal number of
queues. Is there something we should do for mmio? If yes, should this
be a callback in VirtioBusClass?