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

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Message-ID: <20090519095918.GA11790@elte.hu>
Date:	Tue, 19 May 2009 11:59:18 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [GIT PULL] xen /proc/mtrr implementation


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Ingo Molnar wrote:
>> Here Xen invades an already fragile piece of upstream code  
>> (/proc/mtrr) that is obsolete and on the way out. If you want a  
>> solution you should add PAT support to Xen and you should use recent  
>> upstream kernels. Or you should emulate /proc/mtrr in _Xen the  
>> hypervisor_, if you really care that much - without increasing the  
>> amount of crap in Linux.
>>   
>
> That's a gross mis-characterisation of what we're talking about here.
>
> arch/x86 already defines an mtrr_ops, which defines how to 
> manipulate the MTRR registers.  There are currently several 
> implementations of that interface.  In Xen the MTRR registers 
> belong to the hypervisor, but it allows a privileged kernel to 
> modify them via hypercalls.  I simply added a new, straightforward 
> mtrr_ops implementation to do that.  It adds about 120 lines of 
> new code, in a single mtrr/xen.c file.
>
> That's it.  I could add any number of bizarre convolutions to 
> achieve the same effect, but given that there's an existing 
> interface that is exactly designed for what we want to achieve, I 
> have to admit it didn't occur to me to do anything else.

Exactly what is 'bizarre' about using the API defined by the _CPU_ 
already, without adding any ad-hoc hypecall? Catch the dom0 WRMSRs, 
filter out the MTRR indices - that's it.

	Ingo
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