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

[Jamie Lokier <jamie@shareable.org>]

Stefano Stabellini wrote:
> Anthony Liguori wrote:
> 
> > Jamie Lokier wrote:
> >>> Indeed.
> >>> I have a patch that adds bdrv_set_read_only that set bs->read_only = 1
> >>> so that bdrv_open2 can check if the flag is set and act accordingly.
> >>> The problem is that in xen we read whether the device should be opened
> >>> read only in the VM config file, I don't think the user can specify to
> >>> open a drive read-only in qemu.
> >>>     
> >> What's missing is a "ro" sub-option to "-drive".
> >>   
> > 
> > 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.

> > This is not friendly to a user who expects that ro would Just Work.

I don't agree that it's not friendly, because I think it's quite
obvious what behaviour to expect.  Users who don't know what a
read-only disk would do can use virt-manager and that can warn them
when they try to tick the option, or equally likely hide the option so
they can never find it :-)

We already have a user interface for it: chmod.  That's less friendly,
but it's the same result.

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...

> Well, it could Just Work for cdrom.

It would Just Work for several media types (cdrom, floppy), and
for hard disks on certain interfaces (virtio, SCSI, USB).

Yes, virtio, SCSI and USB have read-only storage flags.

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.

-- Jamie