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

[Andrew Lyon <andrew.lyon@xxxxxxxxx>]

On Mon, Jul 27, 2009 at 11:03 AM, Stefano
Stabellini<stefano.stabellini@xxxxxxxxxxxxx> wrote:
> 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 :)
>
>

I still dont understand why it is necessary to loopback mount the iso
image if I can change it without the new image being loopback mounted,
what is the loop device used for?

Andy

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