Re: [RFC PATCH v2 3/8] vhost_net: Emulate link state up if backend doesn't expose it

[Jason Wang]

On Wed, Oct 19, 2022 at 8:52 PM Eugenio Pérez <eperezma@redhat.com> wrote:
>
> At this moment this code path is not reached, but vdpa devices can offer
> VIRTIO_NET_F_STATUS unconditionally.

So I guess what you mean is that, for the parent that doesn't support
VIRTIO_NET_F_STATUS, emulate one for making sure the ANNOUCNE to work.
This is safe since the spec said the driver will assume the link is
always up if without this feature.

> While the guest must assume that
> link is always up by the standard, qemu will set the status bit to 1
> always in this case.
>
> This makes little use by itself, but VIRTIO_NET_F_STATUS is needed for
> the guest to read status bit VIRTIO_NET_F_GUEST_ANNOUNCE, used by feature
> VIRTIO_NET_F_GUEST_ANNOUNCE. So qemu must emulate status feature in case
> it needs to emulate the guest announce feature.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  hw/net/vhost_net.c | 27 ++++++++++++++++++++++++++-
>  1 file changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> index d28f8b974b..5660606c1d 100644
> --- a/hw/net/vhost_net.c
> +++ b/hw/net/vhost_net.c
> @@ -117,7 +117,32 @@ uint64_t vhost_net_get_features(struct vhost_net *net, 
> uint64_t features)
>  int vhost_net_get_config(struct vhost_net *net,  uint8_t *config,
>                           uint32_t config_len)
>  {
> -    return vhost_dev_get_config(&net->dev, config, config_len, NULL);
> +    VirtIODevice *vdev;
> +    int r = vhost_dev_get_config(&net->dev, config, config_len, NULL);
> +
> +    if (unlikely(r != 0)) {
> +        return r;
> +    }
> +
> +    if (config_len < endof(struct virtio_net_config, status)) {
> +        return 0;
> +    }
> +
> +    /*
> +     * TODO: Perform this only if vhost_vdpa.
> +     */

Cindy adds some mediation codes for vhost-vDPA in
virtio_net_get_config(), so I believe it can be done there?

Thanks

> +    vdev = net->dev.vdev;
> +    if (!vdev) {
> +        /* Device is starting */
> +        return 0;
> +    }
> +
> +    if ((net->dev.acked_features & BIT_ULL(VIRTIO_NET_F_STATUS)) &&
> +        !(net->dev.features & BIT_ULL(VIRTIO_NET_F_STATUS))) {
> +        ((struct virtio_net_config *)config)->status |= VIRTIO_NET_S_LINK_UP;
> +    }
> +
> +    return 0;
>  }
>  int vhost_net_set_config(struct vhost_net *net, const uint8_t *data,
>                           uint32_t offset, uint32_t size, uint32_t flags)
> --
> 2.31.1
>