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From:   Eugenio Perez Martin <eperezma@redhat.com>
Date:   Tue, 4 Jul 2023 13:36:11 +0200
Message-ID: <CAJaqyWfdPpkD5pY4tfzQdOscLBcrDBhBqzWjMbY_ZKsoyiqGdA@mail.gmail.com>
Subject: Re: [PATCH] vdpa: reject F_ENABLE_AFTER_DRIVER_OK if backend does not
 support it
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Jason Wang <jasowang@redhat.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Shannon Nelson <shannon.nelson@amd.com>,
        virtualization@lists.linux-foundation.org, kvm@vger.kernel.org
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On Tue, Jul 4, 2023 at 12:38=E2=80=AFPM 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=E2=80=AFPM Michael S. Tsirkin <mst@redhat.c=
om> wrote:
> > >
> > > On Mon, Jul 03, 2023 at 04:22:18PM +0200, Eugenio P=C3=A9rez 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 wil=
l not
> > > > complain for it.
> > > >
> > > > Since there is no specific reason for any parent to reject that bac=
kend
> > > > feature bit when it has been proposed, let's control it at vdpa fro=
ntend
> > > > level. Future patches may move this control to the parent driver.
> > > >
> > > > Fixes: 967800d2d52e ("vdpa: accept VHOST_BACKEND_F_ENABLE_AFTER_DRI=
VER_OK backend feature")
> > > > Signed-off-by: Eugenio P=C3=A9rez <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

> > 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/kerne=
l/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 =3D filep->private_data;
> > > >       struct vhost_dev *d =3D &v->vdev;
> > > > +     const struct vdpa_config_ops *ops =3D v->vdpa->config;
> > > >       void __user *argp =3D (void __user *)arg;
> > > >       u64 __user *featurep =3D argp;
> > > > -     u64 features;
> > > > +     u64 features, parent_features =3D 0;
> > > >       long r =3D 0;
> > > >
> > > >       if (cmd =3D=3D VHOST_SET_BACKEND_FEATURES) {
> > > >               if (copy_from_user(&features, featurep, sizeof(featur=
es)))
> > > >                       return -EFAULT;
> > > > +             if (ops->get_backend_features)
> > > > +                     parent_features =3D 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
> > >
>