Re: [Xen-devel] bi-modal backends - fronend mode detection

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 6/11/06 08:30, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> Hmm, I specifically wanted to try to not touch the front ends at all, so that
> to them the addition of the 32on64 capability is transparent (obviously, soon
> we should want to support migrating a 32-bit domU between a pure 32-bit
> setup and a mixed-mode one, which wouldn't work for pre-existing domU-s
> if they had to report extra info).

For PV guests the tool stack will always know the guest mode and can fill in
xenstore information automatically, both on initial build and on restore. So
it matters not a jot whether the frontends are modified in this case.

The only issue then is for PV-on-HVM drivers, and there's simply not a big
installed base of those yet. The creases are still being ironed out.

> At latest at the point where the front end
> connects it should be possible to reliably determine the guest's mode,
> shouldn't it? Or else, why would you think this is unreliable?

What would you key off? EFER.LME? It would probably work okay, although I'm
sure we could come up with theoretical situations where it wouldn't, but
most guests (including driver domains) are not privileged to obtain the
state of other guests. So this would require a skanky hypercall hack.

 -- Keir



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