Re: [PATCH] vdpa: reject F_ENABLE_AFTER_DRIVER_OK if backend does not support it

[Eugenio Perez Martin <eperezma@redhat.com>]

On Wed, Jul 5, 2023 at 9:50 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Tue, Jul 4, 2023 at 11:45 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Tue, Jul 04, 2023 at 01:36:11PM +0200, Eugenio Perez Martin wrote:
> > > On Tue, Jul 4, 2023 at 12:38 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > On Tue, Jul 04, 2023 at 12:25:32PM +0200, Eugenio Perez Martin wrote:
> > > > > On Mon, Jul 3, 2023 at 4:52 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > > >
> > > > > > On Mon, Jul 03, 2023 at 04:22:18PM +0200, Eugenio Pérez wrote:
> > > > > > > With the current code it is accepted as long as userland send it.
> > > > > > >
> > > > > > > Although userland should not set a feature flag that has not been
> > > > > > > offered to it with VHOST_GET_BACKEND_FEATURES, the current code will not
> > > > > > > complain for it.
> > > > > > >
> > > > > > > Since there is no specific reason for any parent to reject that backend
> > > > > > > feature bit when it has been proposed, let's control it at vdpa frontend
> > > > > > > level. Future patches may move this control to the parent driver.
> > > > > > >
> > > > > > > Fixes: 967800d2d52e ("vdpa: accept VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK backend feature")
> > > > > > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > > > >
> > > > > > Please do send v3. And again, I don't want to send "after driver ok" hack
> > > > > > upstream at all, I merged it in next just to give it some testing.
> > > > > > We want RING_ACCESS_AFTER_KICK or some such.
> > > > > >
> > > > >
> > > > > Current devices do not support that semantic.
> > > >
> > > > Which devices specifically access the ring after DRIVER_OK but before
> > > > a kick?
> > > >
> > >
> > > Previous versions of the QEMU LM series did a spurious kick to start
> > > traffic at the LM destination [1]. When it was proposed, that spurious
> > > kick was removed from the series because to check for descriptors
> > > after driver_ok, even without a kick, was considered work of the
> > > parent driver.
> > >
> > > I'm ok to go back to this spurious kick, but I'm not sure if the hw
> > > will read the ring before the kick actually. I can ask.
> > >
> > > Thanks!
> > >
> > > [1] https://lists.nongnu.org/archive/html/qemu-devel/2023-01/msg02775.html
> >
> > Let's find out. We need to check for ENABLE_AFTER_DRIVER_OK too, no?
>
> My understanding is [1] assuming ACCESS_AFTER_KICK. This seems
> sub-optimal than assuming ENABLE_AFTER_DRIVER_OK.
>
> But this reminds me one thing, as the thread is going too long, I
> wonder if we simply assume ENABLE_AFTER_DRIVER_OK if RING_RESET is
> supported?
>

The problem with that is that the device needs to support all
RING_RESET, like to be able to change vq address etc after DRIVER_OK.
Not all HW support it.

We just need the subset of having the dataplane freezed until all CVQ
commands have been consumed. I'm sure current vDPA code already
supports it in some devices, like MLX and PSD.

Thanks!

> Thanks
>
> >
> >
> >
> > > > > My plan was to convert
> > > > > it in vp_vdpa if needed, and reuse the current vdpa ops. Sorry if I
> > > > > was not explicit enough.
> > > > >
> > > > > The only solution I can see to that is to trap & emulate in the vdpa
> > > > > (parent?) driver, as talked in virtio-comment. But that complicates
> > > > > the architecture:
> > > > > * Offer VHOST_BACKEND_F_RING_ACCESS_AFTER_KICK
> > > > > * Store vq enable state separately, at
> > > > > vdpa->config->set_vq_ready(true), but not transmit that enable to hw
> > > > > * Store the doorbell state separately, but do not configure it to the
> > > > > device directly.
> > > > >
> > > > > But how to recover if the device cannot configure them at kick time,
> > > > > for example?
> > > > >
> > > > > Maybe we can just fail if the parent driver does not support enabling
> > > > > the vq after DRIVER_OK? That way no new feature flag is needed.
> > > > >
> > > > > Thanks!
> > > > >
> > > > > >
> > > > > > > ---
> > > > > > > Sent with Fixes: tag pointing to git.kernel.org/pub/scm/linux/kernel/git/mst
> > > > > > > commit. Please let me know if I should send a v3 of [1] instead.
> > > > > > >
> > > > > > > [1] https://lore.kernel.org/lkml/20230609121244-mutt-send-email-mst@kernel.org/T/
> > > > > > > ---
> > > > > > >  drivers/vhost/vdpa.c | 7 +++++--
> > > > > > >  1 file changed, 5 insertions(+), 2 deletions(-)
> > > > > > >
> > > > > > > diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> > > > > > > index e1abf29fed5b..a7e554352351 100644
> > > > > > > --- a/drivers/vhost/vdpa.c
> > > > > > > +++ b/drivers/vhost/vdpa.c
> > > > > > > @@ -681,18 +681,21 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
> > > > > > >  {
> > > > > > >       struct vhost_vdpa *v = filep->private_data;
> > > > > > >       struct vhost_dev *d = &v->vdev;
> > > > > > > +     const struct vdpa_config_ops *ops = v->vdpa->config;
> > > > > > >       void __user *argp = (void __user *)arg;
> > > > > > >       u64 __user *featurep = argp;
> > > > > > > -     u64 features;
> > > > > > > +     u64 features, parent_features = 0;
> > > > > > >       long r = 0;
> > > > > > >
> > > > > > >       if (cmd == VHOST_SET_BACKEND_FEATURES) {
> > > > > > >               if (copy_from_user(&features, featurep, sizeof(features)))
> > > > > > >                       return -EFAULT;
> > > > > > > +             if (ops->get_backend_features)
> > > > > > > +                     parent_features = ops->get_backend_features(v->vdpa);
> > > > > > >               if (features & ~(VHOST_VDPA_BACKEND_FEATURES |
> > > > > > >                                BIT_ULL(VHOST_BACKEND_F_SUSPEND) |
> > > > > > >                                BIT_ULL(VHOST_BACKEND_F_RESUME) |
> > > > > > > -                              BIT_ULL(VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK)))
> > > > > > > +                              parent_features))
> > > > > > >                       return -EOPNOTSUPP;
> > > > > > >               if ((features & BIT_ULL(VHOST_BACKEND_F_SUSPEND)) &&
> > > > > > >                    !vhost_vdpa_can_suspend(v))
> > > > > > > --
> > > > > > > 2.39.3
> > > > > >
> > > >
> >
>