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

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Date:	Tue, 07 Oct 2008 18:09:57 -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:
> H. Peter Anvin wrote:
>> 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. 
> 
> No, that would be a horrible, horrible mistake.  There's no sane way to 
> implement that; it would mean that the hypervisor would have to have 
> some kind of state model that incorporates all the ABIs in a consistent 
> way.  Any guest using multiple ABIs would effectively end up being 
> dependent on a particular hypervisor via a frankensteinian interface 
> that no other hypervisor would implement in the same way, even if they 
> claim to implement the same set of interfaces.
> 
> If the hypervisor just needs to deal with one at a time then it can have 
> relatively simple ABI<->internal state translation.
> 
> However, if you have the notion of hypervisor-agnostic or common 
> interfaces, then you can include those as part of the rest of the ABI 
> and make it sane (so Xen+common, hyperv+common, etc).
> 

It depends on what classes of interfaces you're talking about.  I think 
you and Jun have a bit narrow definition of "ABI" in this context.  This 
is functionally equivalent to hardware interfaces (after all, that is 
what the hypervisor ABI *is* as far as the kernel is concerned) -- noone 
expects, say, a SATA controller that can run in legacy IDE mode to also 
take AHCI commands at the same time, but the kernel *does* expect that a 
chipset which exports LAPIC, HPET, PMTMR and TSC clock sources can use 
all four at the same time.  In the latter case the interfaces are 
inherently independent and refer to different chunks of hardware which 
just happen to be related in that they all are related to timing.  In 
the former case, we're dealing with *one* piece of hardware which can 
operate in one of two modes.

For hypervisors, you will end up with cases where you have both types -- 
for example, KVM will happily use VMware's video interface, but that 
doesn't mean KVM wants to use VMware's interfaces for storage.  This is 
exactly how it should be: the extent this kind of mix and match that is 
possible is a matter of the definition of the individual interfaces 
themselves, not of the overall architecture.

	-hpa
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