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

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Message-ID: <48E3C32B.3090701@goop.org>
Date:	Wed, 01 Oct 2008 11:36:27 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	akataria@...are.com, "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>,
	Dan Hecht <dhecht@...are.com>,
	Zachary Amsden <zach@...are.com>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org
Subject: Re: [RFC] CPUID usage for interaction between Hypervisors and Linux.

H. Peter Anvin wrote:
> With a sufficiently large block, we could use fixed points, e.g. by 
> having each vendor create interfaces in the 0x40SSSSXX range, where 
> SSSS is the PCI ID they use for PCI devices.

Sure, you could do that, but you'd still want to have a signature in 
0x40SSSS00 to positively identify the chunk.  And what if you wanted 
more than 256 leaves?

> Note that I said "create interfaces".  It's important that all about 
> this is who specified the interface -- for "what hypervisor is this" 
> just use 0x40000000 and disambiguate based on that.

"What hypervisor is this?" isn't a very interesting question; if you're 
even asking it then it suggests that something has gone wrong.  Its much 
more useful to ask "what interfaces does this hypervisor support?", and 
enumerating a smallish range of well-known leaves looking for signatures 
is the simplest way to do that.  (We could use signatures derived from 
the PCI vendor IDs which would help with managing that namespace.)

    J
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