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


From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH] raw-posix.c: remove raw device access for cdrom
Date: Wed, 8 Jul 2015 13:01:40 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

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...

> 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.

Yes, that looks right.

Kevin



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