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

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Message-ID: <48EBE499.5000304@zytor.com>
Date:	Tue, 07 Oct 2008 15:37:13 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Nakajima, Jun" <jun.nakajima@...el.com>
CC:	"akataria@...are.com" <akataria@...are.com>,
	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>,
	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.

Nakajima, Jun wrote:
> On 10/3/2008 5:35:39 PM, H. Peter Anvin wrote:
>> Nakajima, Jun wrote:
>>> What's the significance of supporting multiple interfaces to the
>>> same guest simultaneously, i.e. _runtime_? We don't want the guests
>>> to run on such a literarily Frankenstein machine. And practically,
>>> such testing/debugging would be good only for Halloween :-).
>>>
>> By that notion, EVERY CPU currently shipped is a "Frankenstein" CPU,
>> since at very least they export Intel-derived and AMD-derived interfaces.
>>  This is in other words, a ridiculous claim.
> 
> The big difference here is that you could create a VM at runtime (by combining the existing interfaces) that did not exist before (or was not tested before). For example, a hypervisor could show hyper-v, osx-v (if any), linux-v, etc., and a guest could create a VM with hyper-v MMU, osx-v interrupt handling, Linux-v timer, etc. And such combinations/variations can grow exponentially.
> 
> Or are you suggesting that multiple interfaces be _available_ to guests at runtime but the guest chooses one of them?
> 

The guest chooses what it wants to use.  We already do this: for 
example, we use CPUID leaf 0x80000006 preferentially to CPUID leaf 2, 
simply because it is a better interface.

And you're absolutely right that the guest may end up picking and 
choosing different parts of the interfaces.  That's how it is supposed 
to work.

	-hpa
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