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Re: [Xen-devel] change cd iso image in hvm but loop not changed?

To: Andrew Lyon <andrew.lyon@xxxxxxxxx>
Subject: Re: [Xen-devel] change cd iso image in hvm but loop not changed?
From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date: Mon, 27 Jul 2009 11:03:49 +0100
Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On Sun, 26 Jul 2009, Andrew Lyon wrote:
> Hi,
> I booted a windows hvm with a iso image (/root/t.iso) connected to the
> cdrom, I then changed the iso image using the following technique:
> I used xm block-list <domid> to find the cdrom be-path:
> 5632   0    0     1      -1     -1    /local/domain/0/backend/vbd/3/5632
> To make absolutely sure I got the right device I used xenstore-read:
> xenstore-read /local/domain/0/backend/vbd/3/5632/params
> /root/t.iso
> I then used xenstore-write to change the iso image:
>  xenstore-write /local/domain/0/backend/vbd/3/5632/params
> /home/admin/isos/sql2k8.iso
> After a few seconds the new cd appears in windows, however there is
> still a loop device setup for the original iso image:
> losetup -a
> /dev/loop/0: [0802]:1517 (/root/t.iso)
> Why is this setup? if the cd can be changed without mounting the new
> iso as a loop device why do it in the first place?
> the hvm has gplpv drivers loaded but they do not provide cdrom, it is 
> emulated.

Qemu opens cdrom images directly using open and set a watch on params on
xenstore to recognize if someone changes the cdrom image and if it
happens it closes the old one and open the new one.
On the other hand the vbd backend for the cdrom is set by xend\hotplug scripts
at domain creation so it is never updated afterward.
If you have time to write a patch for xend to better handle cdrom
changes we would be glad to accept it :)

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