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

[Anthony Liguori]

Daniel P. Berrange wrote:
> On Thu, Jul 31, 2008 at 01:26:17PM -0500, Anthony Liguori wrote:
>   
> > So while I think it's valid to have a "read-only disk" exposed to the 
> > guest, I don't think the user should have anything to do with how we 
> > open the file.
> >
> > Is there some specific circumstance you are trying to support?
> >     
>
> The scenario is that the admin wants to assign a read only disk to the
> virtual machine - typically the same disk to multiple machines - and
> thus want to guarentee that no one VM can write to it, since bad things
> happen if you do that with non-cluster filesystems.
>
> Controlling this based on the underlying permissions of the file backing
> the drive is not practical. Things like udev happy set permissions on
> devices in /dev/ behind your back, so you'd have to edit the horrible udev
> config files to make /dev/sdXX readonly.  It is a far simpler task to
> simply add  ,mode=ro  to the QEMU command line for -drive to accomplish
> this, than finding the obscure file to edit to make the underling file
> have read only permissions
>   

Right, but my point is that ,mode=ro does not have to force QEMU to open 
the file O_RDONLY.  It simply needs to prevent writes from happening.

But it's important to be able to expose this property to the guest, so 
,mode=ro should not be allowed for disks that do not support exposing 
their read-only-ness to the guest.

Regards,

Anthony Liguori

> Daniel
>