linux-kernel - Re: [Xen-devel] Re: [GIT PULL] xen /proc/mtrr implementation

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Date:	Tue, 19 May 2009 14:26:23 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Gerd Hoffmann <kraxel@...hat.com>
Cc:	Jan Beulich <JBeulich@...ell.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [Xen-devel] Re: [GIT PULL] xen /proc/mtrr implementation


* Gerd Hoffmann <kraxel@...hat.com> wrote:

> On 05/19/09 13:08, Ingo Molnar wrote:
>> Or, alternatively, the hypervisor can expose its own administrative
>> interface to manage MTRRs.
>
> Guess what?  Xen does exactly that.  And the xen mtrr_ops 
> implementation uses that interface ...

No, that is not an 'administrative interface' - that is a guest 
kernel level hack that complicates Linux, extends its effective ABI 
dependencies and which has to be maintained there from that point 
on.

There's really just three proper technical solutions here:

- either catch the lowlevel CPU hw ops (the MSR modifications, which
  isnt really all that different from the mtrr_ops approach so it
  shouldnt pose undue difficulties to the Xen hypervisor). That will
  be maximally transparent and compatible, with zero changes needed 
  to the Linux kernel.

- or introduce its own hypercall API based administration API,
  without bothering the guest kernel with crap. Trivially patch Xorg
  to make use of it and that's it.

There is absolutely no reason to introduce some intermediate crap 
into Linux.

	Ingo
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