Paolo Bonzini wrote:
I don't see how this fixes anything. If you used feature bits, how do
you migrate from a version that has a feature bit that an older version
doesn't know about? Do you just ignore it?
I'd go with chunk instead of feature bits, specifying them like in the
PNG specification:
You mean, each device would have multiple sections? We already use
chunks for each device state.
Migration needs to be conservative. There should be only two possible
outcomes: 1) a successful live migration or 2) graceful failure with the
source VM still running correctly. Silently ignoring things that could
affect the guests behavior means that it's possible that after failure,
the guest will fail in an unexpected way.
It's up to the source to decide what information is extra. For
example, the state of a RNG emulation is nice-to-have, but as long as
it is initialized from another random source on the destination you
shouldn't care.
We only migrate things that are guest visible. Everything else is left
to the user to configure. We wouldn't migrate the state of a RNG
emulation provided that it doesn't have an impact on the guest.
By definition, anything that is guest visible is important because it
affects the guest's behavior.
Regards,
Anthony Liguori
