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

[Jamie Lokier]

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" :-)

> > Still missing passing the read only attribute to the guest, where possible. 
> > I don't know which device types supports
> > read only, and don't know how to pass this information to guests.

This was discussed in a thread some months ago.  You might look it up.
SCSI, USB, and floppy can pass the information to guests - see the
Linux kernel for how the flag is read.  CD-ROMs are read-only already
of course. I don't know if virtio-blk can; if not, it would be good to
add it.  Not sure about the latest iterations of PATA/SATA.

Read-only images are particularly useful for the backing file for a
qcow2 image or -snapshot.  Then the guest doesn't need to know
anything, but 'savevm', 'delvm' and 'commit' need to be stopped from
trying to modify the backing file.  (But it's fine for -snapshot +
commit to modify a writable qcow2 file which _itself_ has a read-only
backing file, as long as 'commit' doesn't try to push the commit
further back than one level).

> > Also not sure what to do when qemu cannot open the file as
> > writeable. Currently it opens it as read only.  We might change it
> > to give a warning or even an error.

I think that's a good plan, but please don't break the current
situation, where it's possible to "chmod -r" an image file and then
share it safely, until the 'readonly' flag is a usable replacement.

> Is this enough? Basically none of the block drivers know that their
> image could be read-only, so we'll likely trigger some unexpected error
> cases there. For a simple write I guess we'll be okay (not sure if we'll
> return the right error code, though), but I have no idea what, say,
> savevm would do with a read-only image.

Right now there is already a 'readonly' flag called 'chmod -r image',
because QEMU opens a file read-only if it can't open it writable, so
it's not a new case.  Just moving it from the filesystem into QEMU.

When opened read-only, it would be better for the block drivers to
return an error themselves, instead of trying to write and (hopefully)
getting a host OS error.

I doubt if read-only errors have been tested or accomodated, or if
they are reported well to guests as the right sort of error.  I'm
pretty sure the read-only _flag_ isn't passed through guest interfaces
for the guest OS to use yet, but that should be quite easy.

-- Jamie