On 07/14/2017 02:13 PM, Nir Soffer wrote: > Eric, we are more concerned about using the current qemu version. > > We can use the fact that providing both size and backing format, > qemu does not open the backing file, but this is not documented, and > we don't want to base oVirt code on undocumented behavior. > > What we would like to have is: > - qemu blessing for using this undocumented behaviour
But how are you going to get that? By the time someone writes a documentation patch, it won't land until qemu 2.10, but by then, they might as well have written the -u patch instead. Older versions will continue to have the older behavior, unless someone backports a patch to give them the newer behavior - but if someone is backporting -u, presumably they will also backport whatever introspection mechanisms you would also use against upstream qemu to learn if -u is present. If the introspection mechanism is not present or gives the answer that -u is not present, then you can safely assume the old behavior (because the new behavior is only going to be present intentionally). But that is STILL not something that we are going to explicitly document, because it makes more sense to implement the working feature than to document the workaround to a missing feature. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
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