Re: [Qemu-devel] [PATCH 05/18] ide: Turn debug messages into assertions

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 01.06.2011 15:44, schrieb Luiz Capitulino:
>> On Thu, 26 May 2011 18:12:08 -0300
>> Luiz Capitulino <address@hidden> wrote:
>> 
>>> On Thu, 19 May 2011 14:33:19 +0200
>>> Kevin Wolf <address@hidden> wrote:
>>>
>>>> These printfs aren't really debug messages, but clearly indicate a bug if 
>>>> they
>>>> ever become effective.
>>>
>>> Then we have a bug somewhere, starting a VM with:
>>>
>>>  # qemu -hda disks/test.img -enable-kvm -m 1G -cdrom /dev/sr0
>>>
>>> Where the host's CDROM is empty, triggers one of these asserts:
>>>
>>>  qmp-unstable/hw/ide/pci.c:299: bmdma_cmd_writeb: Assertion 
>>> `bm->bus->dma->aiocb == ((void *)0)'
>> 
>> I found out why this is happening. I'm passing '-snapshot' to the 
>> command-line,
>> sorry for not mentioning it (I forgot I was using my devel alias).
>
> And suddenly it's reproducible. :-)
>
> I'll have a look.
>
>> I also found out that /usr/bin/eject in the guest won't work when
>> -snapshot is used. Shouldn't qemu ignore this flag when using cdrom
>> passthrough?
>
> "Won't work" means that it works like with a CD-ROM image? That would be
> what I expect, as you end up having a qcow2 image with -snapshot.

With a qcow2 stacked onto the host_cdrom.  The block layer forwards the
guest's eject only if the top driver has a bdrv_eject() method.  Which
qcow2 doesn't have.  I guess bdrv_eject() fails with -ENOTSUP, and
cmd_start_stop_unit() reports a funny error to the guest.

> Not sure what's the best way of fixing this. Maybe just ignoring
> -snapshot for read-only block devices?

Why not, the combination is pointless.

>                                        Or we could try and forward the
> eject request to the backing file if the format driver doesn't handle it.

For what it's worth, the "raw" driver forwards manually.