Re: [Qemu-devel] read-only IDE drives in qemu

[Markus Armbruster]

Frédéric Dreier <address@hidden> writes:

> Hi,
>
> We are updating some machines in our server farm to Ubuntu natty (KVM /
> Opennebula cluster). Since then we used to have some VM with shared
> read-only IDE disks to mirror software repositories through several isolated
> networks (one-way data diode).
>
> The updated qemu does not allow it. It failed to start the VM with a 'Can't
> use a read-only drive' error. After checking in the qemu git, it seems that
> this has been forbidden by this commit
> '7aa9c811ca0761918a0252d0f923a80224953fa6' :
>
> -----------------------------
> Author: Markus Armbruster <address@hidden>  2010-06-28 19:10:36
> Committer: Kevin Wolf <address@hidden>  2010-07-06 17:05:50
> Parent: c4d74df726cb3791d9f1661b58067df5608b627e (ide: Make ide_init_drive()
> return success)
> Child:  dce9e92834cc4f962e547cae46b73ca559d05b0c (ide: Reject invalid CHS
> geometry)
> Branches: master, remotes/origin/master, remotes/origin/stable-0.13,
> remotes/origin/stable-0.14, remotes/origin/stable-0.15
> Follows: v0.12.0-rc0
> Precedes: v0.13.0-rc0
>
>     ide: Reject readonly drives unless CD-ROM
>
>     drive_init() doesn't permit option readonly for if=ide, but that's
>     worthless: we get it via if=none and -device.
>
>     Signed-off-by: Markus Armbruster <address@hidden>
>     Signed-off-by: Kevin Wolf <address@hidden>
> -----------------------------
>
> I do not understand the comment, if there is a workaround or if it something
> to avoid.
>
> May you help me to understand it?

Real IDE disks can't do read-only.

-drive if=ide rejects readonly unless you also supply media=cdrom, which
turns the drive into a CD-ROM.

When -drive if=none got added, we inadvertedly created a loophole: the
reject covered only if=ide, not if=none when used for a -device
ide-drive.  The commit you quoted fixed that.

Can you use an IDE CD-ROM instead?  Or virtio-blk-pci?