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

[Programmingkid]

On Jun 25, 2015, at 11:14 AM, Programmingkid wrote:

> 
> On Jun 25, 2015, at 2:53 AM, Markus Armbruster wrote:
> 
>> Programmingkid <address@hidden> 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 <address@hidden
>>>>> <mailto:address@hidden>>
>>>>> 
>>>>> 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".
> 
> I think we are going to have to agree to disagree. I have never used the 
> /dev/sr(0 | 1) devices and don't see how they would be effected by this 
> patch. Are you trying to say the /dev/sr(0 | 1) devices *should* be handled 
> by this patch?

Thinking about your question some more, I see what you mean. On Linux /dev/sr0 
refers to the cdrom drive. Also on Linux, the /dev/cdrom link refers to the 
/dev/sr0 device file. So if you just use /dev/cdrom, you are good.