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Date: Thu, 31 Jul 2008 17:01:13 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] PATCH: Control over drive open modes for backing file
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To: qemu-devel@nongnu.org
Cc: Paul Brook <paul@codesourcery.com>

Blue Swirl wrote:
> On 7/31/08, Daniel P. Berrange <berrange@redhat.com> wrote:
> > On Thu, Jul 31, 2008 at 02:46:55PM +0100, Paul Brook wrote:
> >  > > +#define BDRV_O_RDONLY      0x0001 /* Force read-only */
> >  > > +#define BDRV_O_WRONLY      0x0002 /* Force writeable, no fallback */
> >  > > +#define BDRV_O_RDWR        0x0003 /* Try writeable, fallback to read-only
> >  > > */
> >  >
> >  > This is IMHO really misleading.  Normal O_* are not bitflags. The code uses
> >  > these as bitflags sometimes, which means your descriptions are contradictory.
> >
> >
> > One alternative approach I considered would be to not have an explicit
> >  flag for writable, and instead have a flag to explicitly indicate that
> >  fallback to read-only shouldn't be attempted.
> >
> >
> >    #define BDRV_O_RDONLY            0x0001
> >
> >    #define BDRV_O_NO_RO_FALLBACK    0x0002
> >
> >  This would probably make the patch smaller because I won't need to update
> >  all the callers which assume flags of '0'  gives a writable file, falling
> >  back to RO.
> >
> >  Other suggestions welcome too...
> 
> Write-only should mean that only writing is allowed, read access
> should not be needed.

You can't write to most formats unless you can read the metadata.
Flat is the exception, but WRONLY doesn't seem particularly useful.

Whereas read-only floppy images, for example, resemble real hardware.

I would suggest: read-only, read-write, and try-write (traditional behaviour).
Or maybe get rid of the last one.

-- Jamie