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

[Programmingkid]

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?