Re: Disk naming (Was Re: [PATCH] Guest boot loadersupport [1/2])

[Anthony Liguori <aliguori@us.ibm.com>]

Adam Heath wrote:

>Er, no.  The blkback allocates it's own id, which is passed around between
>them.
>
>The blkback them maps the id into a handle structure, which then has a void
>*data(or a union, if you want) that maintains a pointer to a filename, or
>reference to a block device, then a function dispatch table that knows how to
>handle the requests.
>  
>
Are you suggesting this is how it should work?

In xen-unstable, the control tools communicate through a ring-queue (a 
fixed length queue with a maximum message size of 60) to the device 
backends and frontends.  They do not map any memory (besides the ring 
queue).

The virtual block device creation process looks something like this:

1) control tools send a create message through the ring queue to the backend
2) for each virtual block device, control tools send a 
BLKIF_BE_VBD_CREATE message to the backend

This message is fixed length (see 
/usr/include/xen/io/domain_controller.h--blkif_be_vbd_create_t) and has 
two fields for the backend device number (pdevice) and the frontend 
device number (vdevice).

When the frontend boots up, it sends the control tools a series of 
messages.  One of those message contains a share memory handle which the 
controls then pass to the backend.  The backend maps this memory address 
and the frontend and backend use this memory area to do the actual 
operations of the block device.

The control tools can only communicate with the backend (right now) via 
the ring queue.  You can't assume that the tools are in the same domain 
as the backend (so you can't just do an ioctl or something to the kernel).

If you wanted to support passing files, you would have to extend the 
blkif_be_vbd_create_t structure to communicate a filename.  This is the 
problem.

Regards,
Anthony Liguori