linux-kernel - Re: Use CPUID to communicate with the hypervisor.

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Message-ID: <48E14D6E.7030105@zytor.com>
Date:	Mon, 29 Sep 2008 14:49:34 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	"Nakajima, Jun" <jun.nakajima@...el.com>,
	"akataria@...are.com" <akataria@...are.com>,
	Gerd Hoffmann <kraxel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	"avi@...hat.com" <avi@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Zach Amsden <zach@...are.com>, Daniel Hecht <dhecht@...are.com>
Subject: Re: Use CPUID to communicate with the hypervisor.

Jeremy Fitzhardinge wrote:
>>>
>> Unless there is a central authority assigning these, "we" can do all
>> we want, enough people will not pay attention.
>>
>> Basically, there needs to be a standards document that describes the
>> architecture, *and* needs to either have universal buy-in with all the
>> vendors or imposed by an authority with enough clout to do so (Intel
>> might.)
> 
> I think using fixed offsets is unwise, since there's already contention
> for the same leaves.  Making sure that each block of leaves (where a
> block is 16, 256 or some other number of leaves) is self-describing via
> ABI signatures is the only sane way to go.  There's still the issue of
> assigning ABI signatures to vendors, but that's 1) less of an issue, and
> 2) can be self-assigned with very low likelihood of collision.  That way
> a guest can scan that region of leaf space for ABI signatures it
> understand, and can pick and choose among what it finds (but not mix and
> match - that sounds like a course for disaster).

If you can't mix and match, there is no point, since very likely all 
hypervisors will have at least some unique information.

> If we use such a scheme, we can 1) avoid any existing users of that
> space, 2) cleanly delimit a hypervisor-agnostic ABI portion of the leaf
> space, and 3) allow hypervisors to implement multiple ABIs at once.

Yes, see my previous "half-baked" sketch.

	-hpa

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