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

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Message-ID: <48E11751.8070901@zytor.com>
Date:	Mon, 29 Sep 2008 10:58:41 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	akataria@...are.com
CC:	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>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"avi@...hat.com" <avi@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Zach Amsden <zach@...are.com>,
	Daniel Hecht <dhecht@...are.com>,
	"Jun.Nakajima@...el.Com" <Jun.Nakajima@...el.Com>
Subject: Re: Use CPUID to communicate with the hypervisor.

Alok Kataria wrote:
>> Shouldn't you check the hypervisor signature here?
> 
> Nope the whole idea of not checking the hypervisor signature is that we
> should keep this interface generic. 

Unfortunately, given current evidence this is entirely unrealistic.

> So for instance right now, VMware has defined 40000010 leaf, if either
> kvm/xen think it could be useful they could just define that leaf to
> return nonzero value and the kernel will start using it for them.
> Likewise, if in future either kvm/xen come up with a need to define a
> new CPUID leaf they can define the semantics for that leaf, and the
> corresponding kernel side stuff. If VMware, think that this new leaf is
> useful, we can then support that leaf in our hypervisor or return zero
> otherwise.

This is only true if you can also except M$ and other hypervisor vendors 
to stick to it.  So far, hypervisor vendors have hardly shown any 
inclination toward standardization.

Hence I really don't think it is sane.

	-hpa
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