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

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Message-ID: <48EBF49E.9030803@zytor.com>
Date:	Tue, 07 Oct 2008 16:45:34 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	"Nakajima, Jun" <jun.nakajima@...el.com>,
	"akataria@...are.com" <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>,
	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.

Jeremy Fitzhardinge wrote:
>>
>> 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.
> 
> That would be crazy.
> 

Not necessarily, although the example above is extreme.  Redundant 
interfaces is the norm in an evolving platform.

>> Or are you suggesting that multiple interfaces be _available_ to 
>> guests at runtime but the guest chooses one of them?
> 
> Right, that's what I've been suggesting.    I think hypervisors should 
> be able to offer multiple ABIs to guests, but a guest has to commit to 
> using one exclusively (ie, once they start to use one then the others 
> turn themselves off, kill the domain, etc).

Not inherently.  Of course, there may be interfaces which are interently 
or by policy mutually exclusive, but a hypervisor should only export the 
interfaces it wants a guest to be able to use.

This is particularly so with CPUID, which is a *data export* interface, 
it doesn't perform any action.

	-hpa
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