Re: [RFC v2 10/13] vhost: add vhost_kernel_set_vring_enable

[Jason Wang <jasowang@redhat.com>]

在 2021/3/16 上午3:48, Eugenio Pérez 写道:
> This method is already present in vhost-user. This commit adapts it to
> vhost-net, so SVQ can use.
>
> vhost_kernel_set_enable stops the device, so qemu can ask for its status
> (next available idx the device was going to consume). When SVQ starts it
> can resume consuming the guest's driver ring, without notice from the
> latter. Not stopping the device before of the swapping could imply that
> it process more buffers than reported, what would duplicate the device
> action.


Note that it might not be the case of vDPA (virtio) or at least virtio 
needs some extension to achieve something similar like this. One example 
is virtio-pci which forbids 0 to be wrote to queue_enable.

This is another reason to start from vhost-vDPA.


>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>   hw/virtio/vhost-backend.c | 29 +++++++++++++++++++++++++++++
>   1 file changed, 29 insertions(+)
>
> diff --git a/hw/virtio/vhost-backend.c b/hw/virtio/vhost-backend.c
> index 31b33bde37..1ac5c574a9 100644
> --- a/hw/virtio/vhost-backend.c
> +++ b/hw/virtio/vhost-backend.c
> @@ -201,6 +201,34 @@ static int vhost_kernel_get_vq_index(struct vhost_dev *dev, int idx)
>       return idx - dev->vq_index;
>   }
>   
> +static int vhost_kernel_set_vq_enable(struct vhost_dev *dev, unsigned idx,
> +                                      bool enable)
> +{
> +    struct vhost_vring_file file = {
> +        .index = idx,
> +    };
> +
> +    if (!enable) {
> +        file.fd = -1; /* Pass -1 to unbind from file. */
> +    } else {
> +        struct vhost_net *vn_dev = container_of(dev, struct vhost_net, dev);
> +        file.fd = vn_dev->backend;


This can only work with vhost-net devices but not vsock/scsi etc.

Thanks


> +    }
> +
> +    return vhost_kernel_net_set_backend(dev, &file);
> +}
> +
> +static int vhost_kernel_set_vring_enable(struct vhost_dev *dev, int enable)
> +{
> +    int i;
> +
> +    for (i = 0; i < dev->nvqs; ++i) {
> +        vhost_kernel_set_vq_enable(dev, i, enable);
> +    }
> +
> +    return 0;
> +}
> +
>   #ifdef CONFIG_VHOST_VSOCK
>   static int vhost_kernel_vsock_set_guest_cid(struct vhost_dev *dev,
>                                               uint64_t guest_cid)
> @@ -317,6 +345,7 @@ static const VhostOps kernel_ops = {
>           .vhost_set_owner = vhost_kernel_set_owner,
>           .vhost_reset_device = vhost_kernel_reset_device,
>           .vhost_get_vq_index = vhost_kernel_get_vq_index,
> +        .vhost_set_vring_enable = vhost_kernel_set_vring_enable,
>   #ifdef CONFIG_VHOST_VSOCK
>           .vhost_vsock_set_guest_cid = vhost_kernel_vsock_set_guest_cid,
>           .vhost_vsock_set_running = vhost_kernel_vsock_set_running,