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

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Message-ID: <48E3E6DF.8060400@codemonkey.ws>
Date:	Wed, 01 Oct 2008 16:08:47 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	akataria@...are.com
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	"avi@...hat.com" <avi@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Gerd Hoffmann <kraxel@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Nakajima, Jun" <jun.nakajima@...el.com>,
	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.

Alok Kataria wrote:
> On Wed, 2008-10-01 at 11:04 -0700, Jeremy Fitzhardinge wrote:
>   
> 2. Divergence in the interface provided by the hypervisors  : 
> 	The reason we brought up a flat hierarchy is because we think we should
> be moving towards a approach where the guest code doesn't diverge too
> much when running under different hypervisors. That is the guest
> essentially does the same thing if its running on say Xen or VMware.
>
> This design IMO, will take us a step backward to  what we already have
> seen with para virt ops. Each hypervisor (mostly) defines its own cpuid
> block, the guest correspondingly needs to have code to handle each of
> these cpuid blocks, with these blocks will mostly being exclusive.
>   

What's wrong with what we have in paravirt_ops?  Just agreeing on CPUID 
doesn't help very much.  You still need a mechanism for doing hypercalls 
to implement anything meaningful.  We aren't going to agree on a 
hypercall mechanism.  KVM uses direct hypercall instructions, Xen uses a 
hypercall page, VMware uses VMI, Hyper-V uses MSR writes.  We all have 
already defined the hypercall namespace in a certain way.

We've already gone down the road of trying to make standard paravirtual 
interfaces (via virtio).  No one was sufficiently interested in 
collaborating.  I don't see why other paravirtualizations are going to 
be much different.

Regards,

Anthony Liguori
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