[PATCH] vhost-vdpa: filter VIRTIO_F_RING_PACKED feature

Michael S. Tsirkin
2023-06-08T14:23:21Z

On Thu, Jun 08, 2023 at 05:29:58PM +0800, Jason Wang
wrote:
> On Thu, Jun 8, 2023 at 5:21?PM Stefano Garzarella <sgarzare at
redhat.com> wrote:
> >
> > On Thu, Jun 08, 2023 at 05:00:00PM +0800, Jason Wang wrote:
> > >On Thu, Jun 8, 2023 at 4:00?PM Stefano Garzarella <sgarzare at
redhat.com> wrote:
> > >>
> > >> 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 at
amd.com/
> > >
> > >Yes, more below.
> > >
> > >>
> > >> >
> > >> >> 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?
> > >
> > >I don't think we need any filtering since:
> > >
> > >PACKED features has been advertised to userspace via uAPI since
> > >6234f80574d7569444d8718355fa2838e92b158b. Once we relax in uAPI,
it
> > >would be very hard to restrict it again. For the userspace that
tries
> > >to negotiate PACKED:
> > >
> > >1) if it doesn't use SET_VRING_BASE/GET_VRING_BASE, everything
works well
> > >2) if it uses SET_VRING_BASE/GET_VRING_BASE. it might fail or
break silently
> > >
> > >If we backport the fixes to -stable, we may break the application
at
> > >least in the case 1).
> >
> > Okay, I see now, thanks for the details!
> >
> > Maybe instead of "break silently", we can return an explicit
error for
> > SET_VRING_BASE/GET_VRING_BASE in stable branches.
> > But if there are not many cases, we can leave it like that.
>
> A second thought, if we need to do something for stable. is it better
> if we just backport Shannon's series to stable?
>
> >
> > I was just concerned about how does the user space understand that it
> > can use SET_VRING_BASE/GET_VRING_BASE for PACKED virtqueues in a given
> > kernel or not.
>
> My understanding is that if packed is advertised, the application
> should assume SET/GET_VRING_BASE work.
>
> Thanks


Let me ask you this. This is a bugfix yes? What is the appropriate Fixes
tag?

> >
> > Thanks,
> > Stefano
> >