Re: bi-modal backends - fronend mode detection

[Gerd Hoffmann <kraxel@suse.de>]

Keir Fraser wrote:
> Actually this doesn't necessarily work reliably. We don't know what mode the
> HVM guest will happen to be running in when we query it via a hypercall. We
> should add an address-bits field to the frontend device-info directory. This
> can be filled in by frontend drivers, and also by the tools when creating PV
> guests. In the absence of the address-bits field, backend can assume native
> bit width or 32-bit width, whichever seems likely to work with most older
> HVM clients.

We have that issue for PV domains too (32-on-64 starts working now),
which is more problematic as we have both 32bit and 64bit distros in the
wild, which of course don't add info to xenstore indicating the protocol
variant.

For the pv-on-hvm case we maybe have the chance to fix it up in a sane
way before plenty of systems use it we do it quickly, for example by
requiring guests adding the info to the xenstore.  I'd go with some
"protocol" instead of "address bits" field, giving numbers "1" and "2"
to the current 32bit and 64bit structs and leaving the door open to
introduce some new protocol "3" at some point in the future.

cheers,
  Gerd

-- 
Gerd Hoffmann <kraxel@suse.de>
http://www.suse.de/~kraxel/julika-dora.jpeg