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RE: [Xen-devel] [RFC] Hypercalls from HVM guests

To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
Subject: RE: [Xen-devel] [RFC] Hypercalls from HVM guests
From: "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>
Date: Sun, 9 Apr 2006 08:08:24 -0700
Cc: Steve Ofsthun <sofsthun@xxxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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Thread-topic: [Xen-devel] [RFC] Hypercalls from HVM guests
Keir Fraser wrote:
> On 9 Apr 2006, at 14:56, Nakajima, Jun wrote:
>> This is a different question, and I think detecting a virtual device
>> (i.e. virtual block device, NIC) or chipset would be a cleaner way at
>> this point. And that would be proper for the patch that Steve
>> mentioned (we wrote it). The fact that it's running on a hypervisor
>> does not necessarily guarantee presence of such virtual devices (in
>> fact they don't exist today ;-). 
>> If we really need to tell if we are running on a hypervisor at a very
>> early point or even in user-mode, I think CPUID with "an unused
>> index" would be the simplest, but so far I haven't seen any usage
>> models that really require that. If we want to add virtualization
>> hints for processor architectures (e.g. MMU) in guests, it would be
>> needed. 
> Executing hypercalls via an indirection page as we do for
> paravirtualised guests is an attractive idea. That would require more
> than just 'are we AMD or Intel' and it would be nice to have that
> future-proofing level of indirection in the initial implementation. We
> could do that via the PCI device too (e.g., use a BAR) though that
> doesn't seem so clean to me.
>   -- Keir

Yep. We should not use PCI device detection to detect such Xen-specific
MMU features. And we should use PCI device detection to detect a virtual
device because the guest needs to initialize and set up the plumbing
(e.g. interrupt lines) as a device. 

I think such an initial implementation for Xen MMU can use CPUID (with
whatever index), and I think we can come back with a specific value for
the index when a patch is ready.

Intel Open Source Technology Center

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