Re: [PATCH v5 3/5] virtio-scsi: default num_queues to -smp N

[Cornelia Huck]

On Mon,  6 Jul 2020 14:56:48 +0100
Stefan Hajnoczi <stefanha@redhat.com> wrote:

Maybe mention 'pci' in the subject as well?

> Automatically size the number of virtio-scsi-pci, vhost-scsi-pci, and
> vhost-user-scsi-pci request virtqueues to match the number of vCPUs.
> Other transports continue to default to 1 request virtqueue.
> 
> A 1:1 virtqueue:vCPU mapping 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.  The
> maximum number of MSI-X vectors and virtqueues limit are respected.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  include/hw/virtio/virtio-scsi.h |  2 ++
>  hw/core/machine.c               |  3 +++
>  hw/scsi/vhost-scsi.c            |  3 ++-
>  hw/scsi/vhost-user-scsi.c       |  3 ++-
>  hw/scsi/virtio-scsi.c           |  6 +++++-
>  hw/virtio/vhost-scsi-pci.c      | 10 +++++++---
>  hw/virtio/vhost-user-scsi-pci.c | 10 +++++++---
>  hw/virtio/virtio-scsi-pci.c     | 10 +++++++---
>  8 files changed, 35 insertions(+), 12 deletions(-)

(...)

> diff --git a/hw/virtio/virtio-scsi-pci.c b/hw/virtio/virtio-scsi-pci.c
> index 3ff9eb7ef6..fa4b3bfb50 100644
> --- a/hw/virtio/virtio-scsi-pci.c
> +++ b/hw/virtio/virtio-scsi-pci.c
> @@ -46,13 +46,17 @@ static void virtio_scsi_pci_realize(VirtIOPCIProxy 
> *vpci_dev, Error **errp)
>  {
>      VirtIOSCSIPCI *dev = VIRTIO_SCSI_PCI(vpci_dev);
>      DeviceState *vdev = DEVICE(&dev->vdev);
> -    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(vdev);
>      DeviceState *proxy = DEVICE(vpci_dev);
> +    VirtIOSCSIConf *conf = &dev->vdev.parent_obj.conf;
>      char *bus_name;
>  
> +    if (conf->num_queues == VIRTIO_SCSI_AUTO_NUM_QUEUES) {
> +        conf->num_queues =
> +            virtio_pci_optimal_num_queues(VIRTIO_SCSI_VQ_NUM_FIXED);
> +    }
> +
>      if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
> -        vpci_dev->nvectors = vs->conf.num_queues +
> -                             VIRTIO_SCSI_VQ_NUM_FIXED + 1;
> +        vpci_dev->nvectors = conf->num_queues + VIRTIO_SCSI_VQ_NUM_FIXED + 1;
>      }
>  
>      /*

One corner case where the setup may end up being a bit odd is a
situation where nvectors was specified, but num_queues was not, and the
device suddenly ends up with more queues than vectors. But I don't see
a reason why you would want to specify nvectors but not num_queues in a
real word scenario, so I think we can ignore that corner case.

Reviewed-by: Cornelia Huck <cohuck@redhat.com>