Re: [Qemu-devel] Regression opening read-only cdroms

[Avi Kivity]

On 06/16/2009 08:46 PM, Jamie Lokier wrote:
> > > It only is useful if we can expose that read-only attribute to the
> > > guest.  You can't do that consistently with all block devices so the
> > > result would be that you'll fail write operations causing a guest to crash.
> > >        
>
> You should get disk write errors, rather than a simple crash.  Any
> decent operating system will spew a lot of printks, which is a clue.
>    

I don't consider this useful behaviour and see no motivation to support it.

> Users who expect things to just work will be even more surprised that
> "-hda image" where image is read-only does not give any error from
> QEMU, but their guest crashes.  Or that "-hda image" works as usual,
> and their guest crashes, and eventually they discover it's because
> their disk image is not writable, and it's always worked before
> because they were using -snapshot or something like that, and QEMU
> didn't warn them it would be a problem...
>    

I agree, for non-cdroms/floppies, non-backing store files we should 
require write access.


> It is also appropriate for disks that you intent to always mount
> read-only in the guest anyway, even if the interface doesn't have a
> flag, just to protect the image from aberrant guest behaviour.
>    

Maybe -drive ...,snapshot.  The disk will be writable, just not persistent.

--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.