Re: [Qemu-devel] PATCH: Control over drive open modes for backing file

[Daniel P. Berrange]

On Thu, Jul 31, 2008 at 05:01:13PM +0100, Jamie Lokier wrote:
> Blue Swirl wrote:
> > On 7/31/08, Daniel P. Berrange <address@hidden> 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).

Functionally, that's what the implementation actually does - its just bad
flag naming that's confusing things here.

Daniel
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