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

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Message-ID: <48E3D7A0.3000403@codemonkey.ws>
Date:	Wed, 01 Oct 2008 15:03:44 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	akataria@...are.com, kvm@...r.kernel.org,
	the arch/x86 maintainers <x86@...nel.org>,
	Dan Hecht <dhecht@...are.com>,
	LKML <linux-kernel@...r.kernel.org>,
	virtualization@...ts.linux-foundation.org,
	"avi@...hat.com" <avi@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC] CPUID usage for interaction between Hypervisors and Linux.

Jeremy Fitzhardinge wrote:
> Alok Kataria wrote:
> 
> No, we're not getting anywhere.  This is an outright broken idea.  The 
> space is too small to be able to chop up in this way, and the number of 
> vendors too large to be able to do it without having a central oversight.
> 
> The only way this can work is by having explicit positive identification 
> of each group of leaves with a signature.  If there's a recognizable 
> signature, then you can inspect the rest of the group; if not, then you 
> can't.  That way, you can avoid any leaf usage which doesn't conform to 
> this model, and you can also simultaneously support multiple hypervisor 
> ABIs.  It also accommodates existing hypervisor use of this leaf space, 
> even if they currently use a fixed location within it.
> 
> A concrete counter-proposal:

Mmm, cpuid bikeshedding :-)

> The space 0x40000000-0x400000ff is reserved for hypervisor usage.
> 
> This region is divided into 16 16-leaf blocks.  Each block has the 
> structure:
> 
> 0x400000x0:
>     eax: max used leaf within the leaf block (max 0x400000xf)

Why even bother with this?  It doesn't seem necessary in your proposal.

Regards,

Anthony Liguori
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