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

[Markus Armbruster <armbru@redhat.com>]

Programmingkid <programmingkidx@gmail.com> writes:

> On Jun 23, 2015, at 2:06 PM, John Snow wrote:
>
>> 
>> 
>> On 06/23/2015 01:56 PM, Programmingkid wrote:
>>> Fix real cdrom detection so that a real cdrom can actually be used.
>>> 
>>> signed-off-by: John Arbuckle <programmingkidx@gmail.com
>>> <mailto:programmingkidx@gmail.com>>
>>> 
>>> This patch has been tested on Mac OS X host and guest. 
>>> Command used: qemu-system-ppc -cdrom /dev/cdrom
>>> 
>>> Note: I was able to view the files using OpenBIOS, but not on 
>>> Mac OS X. The size of the disc is reported correctly but some
>>> error happens that prevents it from mounting in Mac OS X. This
>>> is probably another bug with QEMU.
>>> 
>>> ---
>>> block.c |    3 ++-
>>> 1 files changed, 2 insertions(+), 1 deletions(-)
>>> 
>>> diff --git a/block.c b/block.c
>>> index dd4f58d..75ccfad 100644
>>> --- a/block.c
>>> +++ b/block.c
>>> @@ -583,7 +583,8 @@ static int find_image_format(BlockDriverState *bs,
>>> const char *filename,
>>>     int ret = 0;
>>> 
>>> 
>>> 
>>>     /* Return the raw BlockDriver * to scsi-generic devices or empty
>>> drives */
>>> -    if (bs->sg || !bdrv_is_inserted(bs) || bdrv_getlength(bs) == 0) {
>>> +    if (bs->sg || !bdrv_is_inserted(bs) || bdrv_getlength(bs) == 0 
>>> +               || strcmp("/dev/cdrom", filename) == 0) {
>>>         *pdrv = &bdrv_raw;
>>>         return ret;
>>>     }
>>> -- 
>>> 1.7.5.4
>>> 
>> 
>> 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.
>
> Before the patch, the bdrv_open_inherit() function would be
> incorrectly called. Its documentation says "Opens a disk image (raw,
> qcow2, vmdk, ...)" meaning only for disk image files (not for real
> media). This patch prevents the bdrv_open_inherit() function from ever
> being called. It sets the pdrv variable to the raw format. This made
> sense to me since a real cdrom is read in the raw format.
>
> A quick test does show the patch works. A real cdrom is successfully
> opened on qemu-system-i386 using a Windows XP guest.

What about /dev/sr0, /dev/sr1, and whatever other names could refer to a
block device without a medium?

Comparing filenames isn't a good way to test "is a block device without
a medium".