Re: [PATCH 1/4] vhost: Re-enable vrings after setting features

[Hanna Czenczek]

On 13.04.23 13:03, Stefan Hajnoczi wrote:
> On Tue, 11 Apr 2023 at 11:05, Hanna Czenczek <hreitz@redhat.com> wrote:
> > If the back-end supports the VHOST_USER_F_PROTOCOL_FEATURES feature,
> > setting the vhost features will set this feature, too.  Doing so
> > disables all vrings, which may not be intended.
> >
> > For example, enabling or disabling logging during migration requires
> > setting those features (to set or unset VHOST_F_LOG_ALL), which will
> > automatically disable all vrings.  In either case, the VM is running
> > (disabling logging is done after a failed or cancelled migration, and
> > only once the VM is running again, see comment in
> > memory_global_dirty_log_stop()), so the vrings should really be enabled.
> > As a result, the back-end seems to hang.
> >
> > To fix this, we must remember whether the vrings are supposed to be
> > enabled, and, if so, re-enable them after a SET_FEATURES call that set
> > VHOST_USER_F_PROTOCOL_FEATURES.
> >
> > It seems less than ideal that there is a short period in which the VM is
> > running but the vrings will be stopped (between SET_FEATURES and
> > SET_VRING_ENABLE).  To fix this, we would need to change the protocol,
> > e.g. by introducing a new flag or vhost-user protocol feature to disable
> > disabling vrings whenever VHOST_USER_F_PROTOCOL_FEATURES is set, or add
> > new functions for setting/clearing singular feature bits (so that
> > F_LOG_ALL can be set/cleared without touching F_PROTOCOL_FEATURES).
> >
> > Even with such a potential addition to the protocol, we still need this
> > fix here, because we cannot expect that back-ends will implement this
> > addition.
> >
> > Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
> > ---
> >   include/hw/virtio/vhost.h | 10 ++++++++++
> >   hw/virtio/vhost.c         | 13 +++++++++++++
> >   2 files changed, 23 insertions(+)
> >
> > diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> > index a52f273347..2fe02ed5d4 100644
> > --- a/include/hw/virtio/vhost.h
> > +++ b/include/hw/virtio/vhost.h
> > @@ -90,6 +90,16 @@ struct vhost_dev {
> >       int vq_index_end;
> >       /* if non-zero, minimum required value for max_queues */
> >       int num_queues;
> > +
> > +    /*
> > +     * Whether the virtqueues are supposed to be enabled (via
> > +     * SET_VRING_ENABLE).  Setting the features (e.g. for
> > +     * enabling/disabling logging) will disable all virtqueues if
> > +     * VHOST_USER_F_PROTOCOL_FEATURES is set, so then we need to
> > +     * re-enable them if this field is set.
> > +     */
> > +    bool enable_vqs;
> > +
> >       /**
> >        * vhost feature handling requires matching the feature set
> >        * offered by a backend which may be a subset of the total
> > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > index a266396576..cbff589efa 100644
> > --- a/hw/virtio/vhost.c
> > +++ b/hw/virtio/vhost.c
> > @@ -50,6 +50,8 @@ static unsigned int used_memslots;
> >   static QLIST_HEAD(, vhost_dev) vhost_devices =
> >       QLIST_HEAD_INITIALIZER(vhost_devices);
> >
> > +static int vhost_dev_set_vring_enable(struct vhost_dev *hdev, int enable);
> > +
> >   bool vhost_has_free_slot(void)
> >   {
> >       unsigned int slots_limit = ~0U;
> > @@ -899,6 +901,15 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
> >           }
> >       }
> >
> > +    if (dev->enable_vqs) {
> > +        /*
> > +         * Setting VHOST_USER_F_PROTOCOL_FEATURES would have disabled all
> Is there a reason to put this vhost-user-specific workaround in
> vhost.c instead of vhost-user.c?

My feeling was that this isn’t really vhost-user-specific.  It just so
happens that vhost-user is the only implementation that has a special
feature that disables all vrings, but I can’t see a reason why that
would be vhost-user-specific.

I mean, vhost_dev_set_vring_enable() looks like it can work for any
vhost implementation, but:
- .vhost_set_vring_enable() is indeed only implemented for vhost-user
- vhost_dev_set_vring_enable() won’t do anything if (for a vhost-user
  back-end) the F_PROTOCOL_FEATURES feature hasn’t been negotiated.

So this looked to me like if .vhost_set_vring_enable() were ever
implemented for anything but vhost-user, it’s quite likely that this too
would be gated behind some feature that auto-disables all vrings.

So this, plus the fact that vhost_dev_set_vring_enable() already has
vhost-user-specific code (while being a vhost.c function), I thought
it’d be best to put this into generic vhost.c code, simply because in
the worst case, it’ll be a no-op on other vhost implementations.

But as it is, functionally, of course I can just put it into
vhost_user_set_features().  (It would be a tiny bit more complicated,
because then I’d have to use vhost_user_set_vring_enable(), which
returns an error if F_PROTOCOL_FEATURES isn’t there, which we’d have to
ignore – vhost_dev_set_vring_enable() does that already for me.)

Hanna