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

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Date:   Thu, 1 Jul 2021 18:26:21 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...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 Thu, Jul 1, 2021 at 3:55 PM Jason Wang <jasowang@...hat.com> wrote:
>
>
> 在 2021/7/1 下午2:50, Yongji Xie 写道:
> > On Wed, Jun 30, 2021 at 5:51 PM Stefan Hajnoczi <stefanha@...hat.com> wrote:
> >> 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).
> >>
> > Oh, yes. I miss 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.
>
>
> I think it's better to extend the get_vq_num_max() per virtqueue.
>
> Currently, vDPA assumes the parent to have a global max size. This seems
> to work on most of the parents but not vp-vDPA (which could be backed by
> QEMU, in that case cvq's size is smaller).
>
> Fortunately, we haven't enabled had cvq support in the userspace now.
>
> I can post the fixes.
>

OK. If so, it looks like we need to support the per-vq configuration.
I wonder if it's better to use something like: VDUSE_CREATE_DEVICE ->
VDUSE_SETUP_VQ -> VDUSE_SETUP_VQ -> ... -> VDUSE_ENABLE_DEVICE to do
initialization rather than only use VDUSE_CREATE_DEVICE.

Thanks,
Yongji