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Date: Mon, 20 Jun 2011 16:40:33 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2] Add support for fd: protocol
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To: Corey Bryant <bryntcor@us.ibm.com>
Cc: libvir-list@redhat.com, Anthony Liguori <aliguori@us.ibm.com>, qemu-devel@nongnu.org, Tyler C Hicks <tchicks@us.ibm.com>

On 06/14/2011 04:31 PM, Corey Bryant wrote:
>    - Starting Qemu with a backing file
>

For this we could tell qemu that a file named "xyz" is available via fd 
n, via an extension of the getfd command.

For example

   (qemu) getfd path="/images/my-image.img"
   (qemu) getfd path="/images/template.img"
   (qemu) drive-add path="/images/my-image.img"

The open() for my-image.img first looks up the name in the getfd 
database, and finds it, so it returns the fd from there instead of 
opening.  It then opens the backing file ("template.img") and looks it 
up again, and finds the second fd from the session.

The result is that open()s are satisfied from the monitor, instead of 
the host kernel, but without reversing the request/reply nature of the 
monitor protocol.

A similar extension could be added to the command line:

   qemu -drive file=fd:4,cache=none -path-alias 
name=/images/template.img,path=fd:5

Here the main image is opened via a fd 4; if it needs template.img, it 
gets shunted to fd 5.

-- 
error compiling committee.c: too many arguments to function