Re: [PATCH v2 2/4] virtio-scsi: default num_queues to -smp N

From: Stefan Hajnoczi <stefanha@gmail.com>
To: Cornelia Huck <cohuck@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <fam@euphon.net>,
	Eduardo Habkost <ehabkost@redhat.com>,
	qemu-block@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>,
	qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v2 2/4] virtio-scsi: default num_queues to -smp N
Date: Wed, 29 Jan 2020 15:44:38 +0000	[thread overview]
Message-ID: <20200129154438.GC157595@stefanha-x1.localdomain> (raw)
In-Reply-To: <20200127141031.6e108839.cohuck@redhat.com>

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On Mon, Jan 27, 2020 at 02:10:31PM +0100, Cornelia Huck wrote:
> On Fri, 24 Jan 2020 10:01:57 +0000
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > @@ -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;
> 
> This now maps the request vqs 1:1 to the vcpus. What about the fixed
> vqs? If they don't really matter, amend the comment to explain that?

The fixed vqs don't matter.  They are typically not involved in the data
path, only the control path where performance doesn't matter.

Stefan

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