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    SubjectRe: [PATCH] vhost-vdpa: filter VIRTIO_F_RING_PACKED feature
    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@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@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@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 tried to look at it, but it looks like we have to backport quite a few
    patches, I wrote a few things here:

    https://lore.kernel.org/virtualization/32ejjuvhvcicv7wjuetkv34qtlpa657n4zlow4eq3fsi2twozk@iqnd2t5tw2an/

    But if you think it's the best way, though, we can take a better look
    at how many patches are to backport and whether it's risky or not.

    >
    >>
    >> 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.
    >

    Same here. So as an alternative to backporting a large set of patches,
    I proposed to completely disable packed for stable branches where
    vhost-vdpa IOCTLs doesn't support them very well.

    Thanks,
    Stefano

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