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

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?


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().


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