Jeremy Fitzhardinge wrote:
I'm starting to lean toward just using 0000. If for no other reason
than the hypercall space is unsharable.
Well, it could be, but it would take affirmative action on the guest's
part. If there's feature bits for each supported hypercall interface,
then you could have a magic MSR to select which interface you want to
use now. That would allow a generic-interface-using guest to probe for
the generic interface at cpuid leaf 0x40001000, use 40001001 to
determine whether the hypercall interface is available, 4000100x to find
the base of the magic msrs, and write appropriate msr to set the desired
hypercall style (and all this can be done without using vmcall, so it
doesn't matter that hypercall interface is initially established).
The main thing keeping me from doing this ATM is what I perceive as lack
of interest in a generic interface. I think it's also a little
premature given that we don't have any features on the plate yet.
However, I don't think that means that we cannot turn KVM's PV into a
generic one. So here's what I propose.
Let's start building the KVM PV interface on 4000 0000. That means that
Xen cannot initially use it but that's okay. Once KVM-lite is merged
and we have some solid features (and other guests start implementing
them), we can also advertise this interface as a "generic interface" by
also supporting the signature on leave 4000 1000 and using the MSR
trickery that you propose.
As long as we all agree not to use 4000 1000 for now, it leaves open the
possibility of having a generic interface in the future.
Regards,
Anthony Liguori
J
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