Re: [Qemu-devel] [PATCH] Add readonly flag to -drive command

[Michael Tokarev]

Anthony Liguori wrote:
> Jamie Lokier wrote:
> > Kevin Wolf wrote:
> >  
> > > Am 12.10.2009 14:47, schrieb Naphtali Sprei:
> > >    
> > > > In order to safely share an image between guests (as read only 
> > > > drive), add a 'readonly' flag
> > > > to the -drive command (qemu command line and monitor).
> >
> > Heh.  I've been sharing images between guests for ages - using "chmod -r" :-)
>
> Were it not for backwards compatibility, I would remove that and have it 
> error out.
>
> I've had multiple people end up with strangely broken guests because 
> they didn't realize the image was read only.

And I had this issue too.

But for me it's still not clear.  Some disk drives out there can be
switched to read-only mode at runtime by using appropriate command.
Certain (USB and especially SD) flash drives can be read-only too
(usually a tiny swith does that).  Even old good floppy drives had
a "write protect" switch.  How the system determines if such devices
are read-only?  For a HDD or a flash drive, I *guess* the drive return
appropriate error message, no?

But qemu - apparently - just ignores writes in this case, which
results in stalled guest which expects some answer to the write
command..  I guess, because it was always the stalls.

/mjt