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

[Jason Wang]

On Thu, Jun 8, 2023 at 5:21 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxx/
> >
> >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

>
> Thanks,
> Stefano
>