[libvirt] [Qemu-devel] live snapshot wiki updated

Eric Blake eblake at redhat.com
Thu Jul 21 13:47:18 UTC 2011

On 07/21/2011 02:40 AM, Jes Sorensen wrote:
>> The security goal of libvirt is to protect the host from a compromised
>> QEMU, therefore QEMU is, by definition, untrusted.
> Well that part goes both ways. By applying this model you are going to
> make it a hard requirement for libvirt to be upgraded with QEMU even for
> smaller updates.

Right now, libvirt only needs the name of the backing file and type. 
How often has the qcow2 file format changed in such a way that that 
particular information has been incompatibly moved around, so that 
clients have to learn a new file format in order to parse the 
information in its new location?  The set of information that libvirt 
needs out of a qcow2 image is relatively small and stable, compared to 
any other changes being made in the rest of the file format, and the 
libvirt folks are more than welcome to help review any qcow2 format 
changes for stability impacts.

Furthermore, you are forgetting that libvirt already ends up upgrading 
to pick up new qemu features all the time, not just for qcow2 layout 
changes.  If you are okay with the feature set supported by an older 
qemu, then you are also okay using an older libvirt that targets just 
that feature set - you only have to upgrade libvirt when talking to a 
newer qemu that requires the use of a newer feature set.  Older libvirt 
can generally talk to newer qemu if qemu made backwards-compatible changes.

Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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