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Date: Wed, 8 Jul 2015 12:31:25 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] raw-posix.c: remove raw device access for
	cdrom
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To: Laurent Vivier <lvivier@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Qemu-block <qemu-block@nongnu.org>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel qemu-devel <qemu-devel@nongnu.org>, Programmingkid <programmingkidx@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, John Snow <jsnow@redhat.com>

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.

Kevin