Re: [Qemu-devel] [PATCH] block: Set cdrom device read only flag

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 02.08.2012 09:20, schrieb Kevin Shanahan:
>> On Thu, Aug 02, 2012 at 02:49:52PM +0930, Kevin Shanahan wrote:
>>> On Thu, Aug 02, 2012 at 11:46:13AM +0930, Kevin Shanahan wrote:
>>>> On Thu, Aug 02, 2012 at 11:02:42AM +0930, Kevin Shanahan wrote:
>>>>> Set the block driver read_only flag for cdrom devices so that
>>>>> qmp_change_blockdev does not attempt to open cdrom files in read-write
>>>>> mode when changing media.
>>>>
>>>> Hrm, this fixes my simple test case using the kvm monitor directly but
>>>> changing media via libvirt still has the same issue (fails for RO
>>>> files, succeeds for writable files).
>>>>
>>>> $ virsh attach-disk --type cdrom --mode readonly test1
>>>> /srv/kvm/iso/ubuntu-12.04-server-amd64.iso hdc
>>>> error: Failed to attach disk
>>>> error: internal error unable to execute QEMU command 'change':
>>>> Could not open '/srv/kvm/iso/ubuntu-12.04-server-amd64.iso'
>>>>
>>>> I'll keep looking into it.
>>>
>>> In the libvirt case, it seems libvirt is failing to add media=cdrom to
>>> the commandline, so in this case qemu is defaulting to media=disk and
>>> my proposed fix has no effect. Diving into libvirt now to see why no
>>> media=disk is getting added...
>>>
>>> Common test case has this xml (generated by virt-install):
>>>
>>>     <disk type='block' device='cdrom'>
>>>       <driver name='qemu' type='raw'/>
>>>       <target dev='hdc' bus='ide'/>
>>>       <readonly/>
>>>       <alias name='ide0-1-0'/>
>>>       <address type='drive' controller='0' bus='1' target='0' unit='0'/>
>>>     </disk>
>> 
>> Ok, looks like libvirt is intentionally leaving media=cdrom off the
>> command line in the case that "-device ide-cd,..." is
>> supported. Presumably by specifying the device this way, qemu is
>> supposed to work out that the media type is cdrom automatically (but
>> it doesn't, it defaults to disk).
>> 
>> Libvirt wants to use:
>> 
>> qemu-kvm ... \
>>      -drive if=none,id=drive-ide0-1-0,readonly=on,format=raw \
>>      -device ide-cd,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 ...
>> 
>> If I hack qemu/qemu_command.c::qemuBuildDriveStr() to ignore the check
>> for QEMU_CAPS_IDE_CD and always add media=cdrom, then (with my qemu as
>> well patch) qemu will open cdrom media files read-only.
>> 
>> There's probably a neater way to just get qemu to set the media type
>> if "-device ide-cd,..." is used, but I haven't worked it out yet.
>> 
>> Anyway, apologies for the rambling conversation with myself on your
>> lists. Hope this is helpful in some way.
>
> Thanks, that's some good information.
>
> However, I don't think you should start from media=cdrom. libvirt
> already does specify readonly=on and that is the property you're really
> interested in. Since commit 528f7663 (released with 0.13) the 'change'
> command should keep the read-only flag for all kinds of media.

Correct.

> Now I'm not sure where you lost the read-only flag. At least on git
> master it doesn't seem to reproduce for me.

I can:

$ qemu --enable-kvm -S -m 384 -vnc localhost:5500 -monitor stdio \
-drive if=none,id=cd,readonly=on,format=raw \
-device ide-cd,bus=ide.1,unit=0,drive=cd
QEMU 1.1.50 monitor - type 'help' for more information
(qemu) change cd r5.iso
Could not open 'r5.iso'
(qemu) q
address@hidden:~/work/images$ ls -l r5.iso 
-r--r--r--. 1 armbru armbru 2872639488 Mar 31  2011 r5.iso

Looks like a QEMU bug to me.