Re: [PATCH RESEND v3 2/4] virtio-scsi: default num_queues to -smp N

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 20 Mar 2020 10:30:39 +0000
Stefan Hajnoczi <stefanha@redhat.com> wrote:

> 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.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  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 ++++++++--
>  include/hw/virtio/virtio-scsi.h |  2 ++
>  8 files changed, 38 insertions(+), 9 deletions(-)

(...)

> diff --git a/hw/virtio/vhost-scsi-pci.c b/hw/virtio/vhost-scsi-pci.c
> index 5dce640eaf..a0b7cdc1ac 100644
> --- a/hw/virtio/vhost-scsi-pci.c
> +++ b/hw/virtio/vhost-scsi-pci.c
> @@ -17,6 +17,7 @@
>  #include "qemu/osdep.h"
>  
>  #include "standard-headers/linux/virtio_pci.h"
> +#include "hw/boards.h"
>  #include "hw/qdev-properties.h"
>  #include "hw/virtio/vhost-scsi.h"
>  #include "qapi/error.h"
> @@ -47,10 +48,15 @@ static void vhost_scsi_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
>  {
>      VHostSCSIPCI *dev = VHOST_SCSI_PCI(vpci_dev);
>      DeviceState *vdev = DEVICE(&dev->vdev);
> -    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(vdev);
> +    VirtIOSCSIConf *conf = &dev->vdev.parent_obj.parent_obj.conf;
> +
> +    /* 1:1 vq to vcpu mapping is ideal because it avoids IPIs */
> +    if (conf->num_queues == VIRTIO_SCSI_AUTO_NUM_QUEUES) {
> +        conf->num_queues = current_machine->smp.cpus;

I don't recall the discussion from previous versions of this patch
set... do we need to bound this by the maximum number of virtqueues? It
seems unlikely that something will break from my reading of the code,
but it might still be nicer.

> +    }
>  
>      if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
> -        vpci_dev->nvectors = vs->conf.num_queues + 3;
> +        vpci_dev->nvectors = conf->num_queues + VIRTIO_SCSI_VQ_NUM_FIXED + 1;

You might already do the resolving of 3 into NUM_FIXED + 1 in the
previous patch; but as you touch this line anyway, I'd just keep this
if you don't need to respin.

>      }
>  
>      qdev_set_parent_bus(vdev, BUS(&vpci_dev->bus));

(Same comments apply to the two other cases below.)