Re: [PATCH] Blktap: Userspace file-based image support. (RFC)

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

Andrew Warfield wrote:
>> A very much like the idea of a userspace block device backend.  Have you
>> considered what it would take to completely replace blkback with a
>> userspace backend?  I'm also curious why you choose a character device
>> to interact with the ring queue instead of just attaching to the ring
>> queue directly in userspace.
>
> Oops (again), missed answering your char device question.  We just use
> a char device to pin up a region of virtual address space for each
> disk as it's presented in userspace.

Is this strictly needed though?  My current understanding (which may be 
totally off) of this device is that it contains:

 - first page is ring/queue
 - rest of file is mmap()'able and as requests come in over the blkfront 
queue, you map them into that address space
 - poll/ioctl is used for event channel notification

Couldn't you do all of this in pure userspace though with privcmd and 
evtchn?

Regards,

Anthony Liguori

>   Anyone familiar with blkback
> will recognise the technique.  In our case, the first page is a
> request/response ring between tap driver and application, and the
> remainder is a sparsely populated address space where data pages are
> mapped as they fly through.  We signal down with ioctl()s, and up
> using poll().
>
> a.