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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 3/11/06 8:43 pm, "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx> wrote:

>> Does anyone have a good suggestion how to have the backends (for
>> which I just created bi-modal functionality, so they can [on x86 for
>> now] support both 32-bit and 64-bit domUs) learn the mode of the
>> connecting frontends.
>> 
>> (Backends using blkif and tpmif need this, pci and networking seem
>> to be unaffected.)
> 
> Maybe using a new hypercall makes sense, to discover what Xen thinks is the
> mode of VCPU0. We'll need something similar for HVM guests in the long term,
> so making the decision at domain-build time doesn't really work.

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.

 -- Keir



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