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

From: Programmingkid
Subject: Re: [Qemu-devel] [PATCH] raw-posix.c: remove raw device access for cdrom
Date: Wed, 8 Jul 2015 08:56:18 -0400

On Jul 8, 2015, at 7:01 AM, Kevin Wolf wrote:

> Am 08.07.2015 um 12:47 hat Laurent Vivier geschrieben:
>> 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.
> Oh, we're already playing such games... I guess you're right then.
> It even seems to be not only for '/dev/cdrom', but for everything
> starting with this string. Does anyone know what's the reason for that?
> Also, I guess before doing strcat() on bsdPath, we should check the
> buffer length...

By buffer, do you mean the bsdPath variable?

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