Re: [Qemu-devel] [PATCH] raw-posix.c: remove raw device access for cdrom

[Laurent Vivier <lvivier@redhat.com>]

On 08/07/2015 12:31, Kevin Wolf wrote:
> Am 02.07.2015 um 16:18 hat Laurent Vivier geschrieben:
>>
>>
>> On 02/07/2015 16:03, Paolo Bonzini wrote:
>>>
>>>
>>> On 02/07/2015 15:58, Laurent Vivier wrote:
>>>> Since any /dev entry can be treated as a raw disk image, it is worth
>>>> noting which devices can be accessed when and how. /dev/rdisk nodes are
>>>> character-special devices, but are "raw" in the BSD sense and force
>>>> block-aligned I/O. They are closer to the physical disk than the buffer
>>>> cache. /dev/disk nodes, on the other hand, are buffered block-special
>>>> devices and are used primarily by the kernel's filesystem code.
>>>
>>> So the right thing to do would not be just to set need_alignment, but to
>>> probe it like we do on Linux for BDRV_O_NO_CACHE.
>>>
>>> I'm okay with doing the simple thing, but it needs a comment for non-BSDers.
>>
>> So, what we have to do, in our case, for MacOS X cdrom, is something like:
>>
>> ... GetBSDPath ...
>> ...
>>     if (flags & BDRV_O_NOCACHE) {
>>         strcat(bsdPath, "r");
>>     }
>> ...
> 
> I would avoid such magic. What we could do is rejecting /dev/rdisk nodes
> without BDRV_O_NOCACHE.

It's not how it works...

Look in hdev_open().

If user provides /dev/cdrom on the command line, in the case of MacOS X,
QEMU searches for a cdrom drive in the system and set filename to
/dev/rdiskX according to the result.

Perhaps this part should be removed.

But if we just want to correct the bug, we must not set filename to
/dev/rdiskX if NOCACHE is not set but to /dev/diskX

It's the aim of this change.

Laurent