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

[Laurent Vivier]

On 25/06/2015 19:19, Laurent Vivier wrote:
> 
> 
> On 25/06/2015 18:16, Paolo Bonzini wrote:
>>
>>
>> On 25/06/2015 18:12, Laurent Vivier wrote:
>>>
>>>
>>> On 25/06/2015 17:48, Paolo Bonzini wrote:
>>>>
>>>> On 25/06/2015 17:32, Programmingkid wrote:
>>>>>> 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.
>>>>
>>>> Well, that's not how things work.
>>>>
>>>> If you do things like that, you end up with a bunch of hacks, not with a
>>>> decent piece of software.
>>>>
>>>> There is support for CD-ROM passthrough on Linux and FreeBSD in
>>>> block/raw-posix.c.  Perhaps the FreeBSD support can be extended to OS X
>>>> as well.
>>>>
>>>
>>> In fact, programmingkid, you should fix it in hdev_open() where there is
>>> already a #if __APPLE__ .
>>>
>>> Paolo, I agree with you but :
>>>
>>> hdev_open()
>>>
>>> #if defined(__linux__)
>>>     {
>>>         char resolved_path[ MAXPATHLEN ], *temp;
>>>
>>>         temp = realpath(filename, resolved_path);
>>>         if (temp && strstart(temp, "/dev/sg", NULL)) {
>>>             bs->sg = 1;
>>>         }
>>> #endif
>>>
>>> I'm wondering who had this strange idea... :)
>>
>> I was very scared to type "git blame" here. :)  But the question is also
> 
> http://geek-and-poke.com/2013/11/24/simply-explained
> 
> BTW, it is a legacy from 2006:
> 
> 19cb373 better support of host drives
> 
> coming from MacOS X (again!):
> 
> 3b0d4f6 OS X: support for the built in CD-ROM drive (Mike Kronenberg)
> 
>> where to put the checks.  Putting it at a random place in block.c is not
>> a good idea.
>>
>> But yes, this is also bad.  It should use stat and check the major/minor
>> numbers.
> 
> Yes, we should check if major is SCSI_GENERIC_MAJOR (21) (on linux).
> 
> We can also try to send an SG command like in cdrom_probe_device().
> Something like in scsi_generic_realize():
> 
>     rc = blk_ioctl(s->conf.blk, SG_GET_VERSION_NUM, &sg_version);
>     if (rc < 0) {
>         error_setg(errp, "cannot get SG_IO version number: %s.  "
>                          "Is this a SCSI device?",
>                          strerror(-rc));
>         return;
>     }
> 

BTW, this solution is already queued in Stefan's block tree:

https://github.com/stefanha/qemu/commit/3307ed7b3fac5ba99eb3b84904b0b7cdc3592a61

Laurent