Re: [Qemu-devel] [PATCH] virtio-blk: Allow startup of empty cdroms

[Michal Privoznik <mprivozn@redhat.com>]

On 07.01.2016 05:48, Stefan Hajnoczi wrote:
> On Wed, Jan 06, 2016 at 11:35:43AM +0100, Michal Privoznik wrote:
>> If you have an empty IDE cdrom we will start just fine:
>>
>> -drive if=none,id=drive-ide0-0-0,readonly=on
>> -device ide-cd,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0
>>
>> However, that's not the case with virtio disk:
>>
>> -drive if=none,media=cdrom,id=drive-virtio-disk1,readonly=on
>> -device virtio-blk-pci,scsi=off,bus=pci.2,addr=0x2,drive=drive-virtio-disk1,id=virtio-disk1
>>
>> One will get the following error:
>>
>> qemu-system-x86_64: -device virtio-blk-pci,scsi=off,bus=pci.2,addr=0x2,drive=drive-virtio-disk1,id=virtio-disk1: Device needs media, but drive is empty
>>
>> The error comes from virtio_blk_device_realize() where we check
>> if virtio block device has a media inserted. This should,
>> however, be not required for cdroms.
>>
>> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
>> ---
>>  hw/block/virtio-blk.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> virtio-blk doesn't do CD-ROM emulation but it supports read-only block
> devices.  The difference is that read-only block devices don't offer the
> features that CD-ROMs support like media status and tray eject.
> 
> Also, virtio-blk does not support media change.  If we allow it to start
> with an empty disk, is there an expectation that media can be changed
> later?
> 
> In order to support empty media that can later be inserted virtio-blk.c
> needs to implement BlockDevOps->change_media_cb() to notify the guest.
> You'd need to check guest whether existing guest drivers handle a virtio
> configuration interrupt properly (i.e. by invalidating cached pages from
> the block devices).
> 
> Is this patch supposed to make virtio-blk support CD-ROMs?  In that case
> a lot more work is necessary and it's probably better to use virtio-scsi
> instead.

Yeah, I realized I was aiming at CD-ROMs. But I'm not that familiar with
qemu to supply proper patch. It's just that after 39c4ae941ed992a3bb5 I
had to fix libvirt, because it was passing format= even for sourceless
devices. But while testing it I've noticed that virtio CD-ROMs still
don't work. This was my attempt to fix it. Well, maybe one day :)

Thanks!

Michal