linux-kernel - Re: ABI coupling to hypervisors via CONFIG

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Message-ID: <45F1E160.1090608@goop.org>
Date:	Fri, 09 Mar 2007 14:36:16 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Chris Wright <chrisw@...s-sol.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Zachary Amsden <zach@...are.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	john stultz <johnstul@...ibm.com>, akpm@...ux-foundation.org,
	LKML <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@...e.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: ABI coupling to hypervisors via CONFIG_PARAVIRT

Ingo Molnar wrote:
> ok, sure, how about the one i mentioned: long-term i'd like to have a 
> paravirt model where the guest does not store /any/ page tables - all 
> paging is managed by the hypervisor. The guest has a vma tree, but 
> otherwise it does not process pagefaults, has no concept of a pte (if in 
> paravirt mode), has no concept of kernel page tables either: there are 
> hypercalls to allocate/free guest-kernel memory, etc. This needs some 
> (serious) MM surgery but it's doable and it's interesting as well. How 
> would you map this to the VMI backend?

You wouldn't.  Why would you?  It might be a useful interface - and its
the perfect kind of high-level interface for pv_ops.  It might be worth
adapting a hypervisor to suit it, but you still need to support the
i386's pagetables.  So, you present the pv_ops interface with your
vma-based mappings, and it runs it through the vma->pagetable
translation layer to feed into either the i386 pagetables directly, or
to a hypervisor's page-based interface.

The important part is that there's more to the story than just pv_ops. 
If you wanted to make such a change, then you'd need to refactor the
i386 support code to add a vma->paging helper layer.  That layer would
be available for any pv_ops interface to use if it wishes.

(Remember, in the pv_ops model, bare hardware is a "hypervisor" too.)

Next problem?

    J
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