netdev - Re: Re: [PATCH v8 09/10] vduse: Introduce VDUSE

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Date:   Wed, 30 Jun 2021 10:51:39 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Yongji Xie <xieyongji@...edance.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Parav Pandit <parav@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christian Brauner <christian.brauner@...onical.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, bcrl@...ck.org,
        Jonathan Corbet <corbet@....net>,
        Mika Penttilä <mika.penttila@...tfour.com>,
        Dan Carpenter <dan.carpenter@...cle.com>, joro@...tes.org,
        Greg KH <gregkh@...uxfoundation.org>, songmuchun@...edance.com,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev@...r.kernel.org, kvm <kvm@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in
 Userspace

On Tue, Jun 29, 2021 at 10:59:51AM +0800, Yongji Xie wrote:
> On Mon, Jun 28, 2021 at 9:02 PM Stefan Hajnoczi <stefanha@...hat.com> wrote:
> >
> > On Tue, Jun 15, 2021 at 10:13:30PM +0800, Xie Yongji wrote:
> > > +/* ioctls */
> > > +
> > > +struct vduse_dev_config {
> > > +     char name[VDUSE_NAME_MAX]; /* vduse device name */
> > > +     __u32 vendor_id; /* virtio vendor id */
> > > +     __u32 device_id; /* virtio device id */
> > > +     __u64 features; /* device features */
> > > +     __u64 bounce_size; /* bounce buffer size for iommu */
> > > +     __u16 vq_size_max; /* the max size of virtqueue */
> >
> > The VIRTIO specification allows per-virtqueue sizes. A device can have
> > two virtqueues, where the first one allows up to 1024 descriptors and
> > the second one allows only 128 descriptors, for example.
> >
> 
> Good point! But it looks like virtio-vdpa/virtio-pci doesn't support
> that now. All virtqueues have the same maximum size.

I see struct vpda_config_ops only supports a per-device max vq size:
u16 (*get_vq_num_max)(struct vdpa_device *vdev);

virtio-pci supports per-virtqueue sizes because the struct
virtio_pci_common_cfg->queue_size register is per-queue (controlled by
queue_select).

I guess this is a question for Jason: will vdpa will keep this limitation?
If yes, then VDUSE can stick to it too without running into problems in
the future.

> > > +     __u16 padding; /* padding */
> > > +     __u32 vq_num; /* the number of virtqueues */
> > > +     __u32 vq_align; /* the allocation alignment of virtqueue's metadata */
> >
> > I'm not sure what this is?
> >
> 
>  This will be used by vring_create_virtqueue() too.

If there is no official definition for the meaning of this value then
"/* same as vring_create_virtqueue()'s vring_align parameter */" would
be clearer. That way the reader knows what to research in order to
understand how this field works.

I don't remember but maybe it was used to support vrings when the
host/guest have non-4KB page sizes. I wonder if anyone has an official
definition for this value?

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