Re: [PATCH for-9.0 v3] vdpa-dev: Fix initialisation order to restore VDUSE compatibility

[Jason Wang <jasowang@redhat.com>]

On Fri, Mar 15, 2024 at 11:59 PM Kevin Wolf <kwolf@redhat.com> wrote:
>
> VDUSE requires that virtqueues are first enabled before the DRIVER_OK
> status flag is set; with the current API of the kernel module, it is
> impossible to enable the opposite order in our block export code because
> userspace is not notified when a virtqueue is enabled.
>
> This requirement also mathces the normal initialisation order as done by
> the generic vhost code in QEMU. However, commit 6c482547 accidentally
> changed the order for vdpa-dev and broke access to VDUSE devices with
> this.
>
> This changes vdpa-dev to use the normal order again and use the standard
> vhost callback .vhost_set_vring_enable for this. VDUSE devices can be
> used with vdpa-dev again after this fix.
>
> vhost_net intentionally avoided enabling the vrings for vdpa and does
> this manually later while it does enable them for other vhost backends.
> Reflect this in the vhost_net code and return early for vdpa, so that
> the behaviour doesn't change for this device.
>
> Cc: qemu-stable@nongnu.org
> Fixes: 6c4825476a4351530bcac17abab72295b75ffe98
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
> v2:
> - Actually make use of the @enable parameter
> - Change vhost_net to preserve the current behaviour
>
> v3:
> - Updated trace point [Stefano]
> - Fixed typo in comment [Stefano]
>
>  hw/net/vhost_net.c     | 10 ++++++++++
>  hw/virtio/vdpa-dev.c   |  5 +----
>  hw/virtio/vhost-vdpa.c | 29 ++++++++++++++++++++++++++---
>  hw/virtio/vhost.c      |  8 +++++++-
>  hw/virtio/trace-events |  2 +-
>  5 files changed, 45 insertions(+), 9 deletions(-)
>
> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> index e8e1661646..fd1a93701a 100644
> --- a/hw/net/vhost_net.c
> +++ b/hw/net/vhost_net.c
> @@ -541,6 +541,16 @@ int vhost_set_vring_enable(NetClientState *nc, int enable)
>      VHostNetState *net = get_vhost_net(nc);
>      const VhostOps *vhost_ops = net->dev.vhost_ops;
>
> +    /*
> +     * vhost-vdpa network devices need to enable dataplane virtqueues after
> +     * DRIVER_OK, so they can recover device state before starting dataplane.
> +     * Because of that, we don't enable virtqueues here and leave it to
> +     * net/vhost-vdpa.c.
> +     */
> +    if (nc->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> +        return 0;
> +    }

I think we need some inputs from Eugenio, this is only needed for
shadow virtqueue during live migration but not other cases.

Thanks