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

  • To: "Anthony Liguori" <aliguori@xxxxxxxxxx>
  • From: "Andrew Warfield" <andrew.warfield@xxxxxxxxxxxx>
  • Date: Mon, 19 Jun 2006 12:26:02 -0700
  • Cc: Xen Developers <xen-devel@xxxxxxxxxxxxxxxxxxx>, Julian Chesterfield <julian.chesterfield@xxxxxxxxxxxx>
  • Delivery-date: Mon, 19 Jun 2006 12:26:26 -0700
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  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

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.  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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