Re: [Qemu-devel] [PATCH v4 for-4.0 2/7] vhost-user: Support transferring inflight buffer between qemu and backend

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jan 09, 2019 at 07:27:23PM +0800, elohimes@gmail.com wrote:
> @@ -382,6 +397,30 @@ If VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD protocol feature is negotiated,
>  slave can send file descriptors (at most 8 descriptors in each message)
>  to master via ancillary data using this fd communication channel.
>  
> +Inflight I/O tracking
> +---------------------
> +
> +To support slave reconnecting, slave need to track inflight I/O in a
> +shared memory. VHOST_USER_GET_INFLIGHT_FD and VHOST_USER_SET_INFLIGHT_FD
> +are used to transfer the memory between master and slave. And to encourage
> +consistency, we provide a recommended format for this memory:

I think we should make a stronger statement and actually
just say what the format is. Not recommend it weakly.

> +
> +offset	 width	  description
> +0x0      0x400    region for queue0
> +0x400    0x400    region for queue1
> +0x800    0x400    region for queue2
> +...      ...      ...
> +
> +For each virtqueue, we have a 1024 bytes region.


Why is the size hardcoded? Why not a function of VQ size?


> The region's format is like:
> +
> +offset   width    description
> +0x0      0x1      descriptor 0 is in use or not
> +0x1      0x1      descriptor 1 is in use or not
> +0x2      0x1      descriptor 2 is in use or not
> +...      ...      ...
> +
> +For each descriptor, we use one byte to specify whether it's in use or not.
> +
>  Protocol features
>  -----------------
> 

I think that it's a good idea to have a version in this region.
Otherwise how are you going to handle compatibility when
this needs to be extended?