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

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Date:	Tue, 19 May 2009 15:21:08 +0200
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
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

On 05/19/09 14:26, Ingo Molnar wrote:
> * 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).

Devil is in the details.

The dom0 kernel might not see all physical cpus on the system.  So Xen 
can't leave the job of looping over all cpus to the dom0 kernel, Xen has 
to apply the changes made by the (priviledged) guest kernel on any 
(virtual) cpu to all (physical) cpus in the machine.

Which in turn means the "lowlevel cpu hw op" would work in a slightly 
different way on Xen and native.  Nasty.

>    That will
>    be maximally transparent and compatible, with zero changes needed
>    to the Linux kernel.

No, the linux kernel probably should do the wrmsr on one cpu only then.

> - 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.

I have serious doubts that this is going to fly with KMS.

Oops, the third "proper technical solutions" is missing.

cheers,
   Gerd

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