Re: [Qemu-block] [PATCH] block.c: fix real cdrom detection

[Stefan Hajnoczi]

On Fri, Jun 26, 2015 at 4:50 PM, Programmingkid
<address@hidden> wrote:
>
> On Jun 26, 2015, at 5:34 AM, Stefan Hajnoczi wrote:
>
>> On Thu, Jun 25, 2015 at 11:11:24AM -0400, Programmingkid wrote:
>>> On Jun 25, 2015, at 9:31 AM, Stefan Hajnoczi wrote:
>>>> On Tue, Jun 23, 2015 at 02:26:51PM -0400, Programmingkid wrote:
>>>>> On Jun 23, 2015, at 2:06 PM, John Snow wrote:
>>>>>> So what's the issue that this patch attempts to fix and how did you
>>>>>> determine that the fix was needed here? It doesn't look like it respects
>>>>>> proper abstraction at a glance.
>>>>>
>>>>> Without the patch, QEMU would just quit when the "-cdrom /dev/cdrom" 
>>>>> option is given.
>>>>
>>>> Why does it quit?
>>>
>>> Because of a bug. This is what it prints: "Could not read image for 
>>> determining its format: Invalid argument".
>>
>> The bdrv_pread() failure is what need you need to investigate.  In the
>> other sub-thread there have been hints about adding CD-ROM passthrough
>> support on Mac OS X by filling in the missing parts in
>> block/raw-posix.c.  That should help you get to the bottom of the
>> problem.
>>
>>> This message seems to indicate that QEMU thinks the real cdrom drive is an 
>>> image file.
>>
>> If the -drive format= option is not given, QEMU will try to detect the
>> image format.  That's the purpose of the find_image_format() function.
>>
>> QEMU does not make a distinction between image files and block devices
>> because there are valid use cases where a block device uses an image
>> format.  For example, a disk or partition can contain qcow2 data (there
>> is no partition table or file system, just qcow2).
>
> So what you are saying is if the user enters "-cdrom /dev/cdrom", QEMU is 
> suppose to call find_image_format. If everything goes right, the first if 
> statement should be skipped. Then the bdrv_pread() function should succeed 
> and so should the bdrv_probe_all() function. Is that how it is suppose to 
> work?

Exactly.  bdrv_pread() will grab some data from the start of the
CD-ROM.  No qcow2, vmdk, etc header will be found (there is probably
an ISO file system there), so QEMU will default to the raw format and
everything will work as expected.  This is also how CD-ROM passthrough
works on Linux and FreeBSD.