linux-kernel - Re: [RFC] CPUID usage for interaction between Hypervisors and Linux.

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Message-ID: <48E3FDD5.7040106@zytor.com>
Date:	Wed, 01 Oct 2008 15:46:45 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	akataria@...are.com
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	"avi@...hat.com" <avi@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Gerd Hoffmann <kraxel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Nakajima, Jun" <jun.nakajima@...el.com>,
	Daniel Hecht <dhecht@...are.com>,
	Zach Amsden <zach@...are.com>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC] CPUID usage for interaction between Hypervisors	and	Linux.

Alok Kataria wrote:
>> No, that's always a terrible idea.  Sure, its necessary to deal with
>> some backward-compatibility issues, but we should even consider a new
>> interface which assumes this kind of thing.  We want properly enumerable
>> interfaces.
> 
> The reason we still have to do this is because, Microsoft has already
> defined a CPUID format which is way different than what you or I are
> proposing ( with the current case of 256 leafs being available). And I
> doubt they would change the way they deal with it on their OS. 
> Any proposal that we go with, we will have to export different CPUID
> interface from the hypervisor for the 2 OS in question. 
> 
> So i think this is something that we anyways will have to do and not
> worth binging about in the discussion.

No, that's a good hint that what "you and I" are proposing is utterly 
broken and exactly underscores what I have been stressing about 
noncompliant hypervisors.

All I have seen out of Microsoft only covers CPUID levels 0x40000000 as 
an vendor identification leaf and 0x40000001 as a "hypervisor 
identification leaf", but you might have access to other information.

This further underscores my belief that using 0x400000xx for anything 
"standards-based" at all is utterly futile, and that this space should 
be treated as vendor identification and the rest as vendor-specific. 
Any hope of creating a standard that's actually usable needs to be 
outside this space, e.g. in the 0x40SSSSxx space I proposed earlier.

	-hpa
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