Re: [Qemu-devel] [PATCH 2/5] Add vhost-kernel and the vhost-user skeleton

[Stefan Hajnoczi]

On Fri, Nov 29, 2013 at 08:52:23PM +0100, Antonios Motakis wrote:
> diff --git a/hw/virtio/vhost-backend.c b/hw/virtio/vhost-backend.c
> index 05de174..80defe4 100644
> --- a/hw/virtio/vhost-backend.c
> +++ b/hw/virtio/vhost-backend.c
> @@ -25,9 +25,18 @@ static int vhost_kernel_call(struct vhost_dev *dev, 
> unsigned long int request,
>  
>  int vhost_call(struct vhost_dev *dev, unsigned long int request, void *arg)
>  {
> -    int result;
> +    int result = -1;
>  
> -    result = vhost_kernel_call(dev, request, arg);
> +    switch (dev->backend_type) {
> +    case VHOST_BACKEND_TYPE_KERNEL:
> +        result = vhost_kernel_call(dev, request, arg);
> +        break;
> +    case VHOST_BACKEND_TYPE_USER:
> +        fprintf(stderr, "vhost-user not implemented\n");
> +        break;
> +    default:
> +        fprintf(stderr, "Unknown vhost backend type\n");
> +    }

The switch statement approach gets messy fast when local variables are
needed inside some case labels.  It also makes it hard to conditionally
compile features without using #ifdef.

Perhaps instead a VhostOps struct could be used:

/* Vhost backends implement this interface */
typedef struct {
    int vhost_call(struct vhost_dev *dev,
                   unsigned long int request,
                   void *arg);
    ...
} VhostOps;

const VhostOps vhost_kernel_ops = {
    ...
};

const VhostOps vhost_user_opts = {
    ...
};

ret = dev->vhost_ops->vhost_call(dev, request, arg);

Something along those lines.  It keeps the different backend
implementations separate (they can live in separate files and be
conditional in Makefile.objs).