Re: [PATCH] vhost-vdpa: filter VIRTIO_F_RING_PACKED feature

[Stefano Garzarella]

On Thu, Jun 08, 2023 at 03:46:00PM +0800, Jason Wang wrote:

[...]

> > > > > > I have a question though, what if down the road there
> > > > > > is a new feature that needs more changes? It will be
> > > > > > broken too just like PACKED no?
> > > > > > Shouldn't vdpa have an allowlist of features it knows how
> > > > > > to support?
> > > > >
> > > > > It looks like we had it, but we took it out (by the way, we were
> > > > > enabling packed even though we didn't support it):
> > > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6234f80574d7569444d8718355fa2838e92b158b
> > > > >
> > > > > The only problem I see is that for each new feature we have to modify
> > > > > the kernel.
> > > > > Could we have new features that don't require handling by vhost-vdpa?
> > > > >
> > > > > Thanks,
> > > > > Stefano
> > > >
> > > > Jason what do you say to reverting this?
> > >
> > > I may miss something but I don't see any problem with vDPA core.
> > >
> > > It's the duty of the parents to advertise the features it has. For example,
> > >
> > > 1) If some kernel version that is packed is not supported via
> > > set_vq_state, parents should not advertise PACKED features in this
> > > case.
> > > 2) If the kernel has support packed set_vq_state(), but it's emulated
> > > cvq doesn't support, parents should not advertise PACKED as well
> > >
> > > If a parent violates the above 2, it looks like a bug of the parents.
> > >
> > > Thanks
> >
> > Yes but what about vhost_vdpa? Talking about that not the core.
>
> Not sure it's a good idea to workaround parent bugs via vhost-vDPA.

Sorry, I'm getting lost...
We were talking about the fact that vhost-vdpa doesn't handle
SET_VRING_BASE/GET_VRING_BASE ioctls well for packed virtqueue before
that series [1], no?

The parents seem okay, but maybe I missed a few things.

[1] https://lore.kernel.org/virtualization/20230424225031.18947-1-shannon.nelson@xxxxxxx/

> > Should that not have a whitelist of features
> > since it interprets ioctls differently depending on this?
>
> If there's a bug, it might only matter the following setup:
>
> SET_VRING_BASE/GET_VRING_BASE + VDUSE.
>
> This seems to be broken since VDUSE was introduced. If we really want
> to backport something, it could be a fix to filter out PACKED in
> VDUSE?

mmm it doesn't seem to be a problem in VDUSE, but in vhost-vdpa.
I think VDUSE works fine with packed virtqueue using virtio-vdpa
(I haven't tried), so why should we filter PACKED in VDUSE?

Thanks,
Stefano