Re: [PATCH 0/3][RFC] virtio-blk: add io_uring passthrough support for virtio-blk

[Jason Wang]

On Tue, Dec 17, 2024 at 12:14 AM Stefan Hajnoczi <stefanha@xxxxxxxxx> wrote:
>
> On Mon, 16 Dec 2024 at 10:54, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> >
> > Hacking passthrough into virtio_blk seems like not very good layering.
> > If you have a use case where you want to use the core kernel virtio code
> > but not the protocol drivers we'll probably need a virtqueue passthrough
> > option of some kind.
>
> I think people are finding that submitting I/O via uring_cmd is faster
> than traditional io_uring. The use case isn't really passthrough, it's
> bypass :).
>
> That's why I asked Jens to weigh in on whether there is a generic
> block layer solution here. If uring_cmd is faster then maybe a generic
> uring_cmd I/O interface can be defined without tying applications to
> device-specific commands. Or maybe the traditional io_uring code path
> can be optimized so that bypass is no longer attractive.
>
> The virtio-level virtqueue passthrough idea is interesting for use
> cases that mix passthrough applications with non-passthrough
> applications. VFIO isn't enough because it prevents sharing and
> excludes non-passthrough applications. Something similar to  VDPA
> might be able to pass through just a subset of virtqueues that
> userspace could access via the vhost_vdpa driver.

I thought it could be reused as a mixing approach like this. The vDPA
driver might just do a shadow virtqueue so in fact we just replace
io_uring here with the virtqueue. Or if we think vDPA is heavyweight,
vhost-blk could be another way.

> This approach
> doesn't scale if many applications are running at the same time
> because the number of virtqueues is finite and often the same as the
> number of CPUs.
>
> Stefan
>

Thanks