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

[Stefan Hajnoczi <stefanha@gmail.com>]

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?

Stefan