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

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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):
> > >
> > >
> > > 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.


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:


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

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?


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