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

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Date:	Wed, 20 May 2009 09:12:07 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
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

Ingo Molnar wrote:
>> 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.
>   

Well, the x86 world can't seem to decide what the ABI is supposed to be, 
which is why we have mtrr_ops in the first place.  Doing emulation at 
the MSR level means that I'd need to decide which MTRR interface we're 
emulating today and do that.

Yes, I realize that almost everyone is using the same Intel-like 
interface these days, but it does mean there's a level of fragility that 
doesn't exist if we just implement mtrr_ops.

There's some secondary issues which arise.  For example, the mtrr 
trimming test is meaningless in dom0 (the e820 is fake, so it doesn't 
make sense to compare it with the mtrrs); we currently avoid that 
because the test only happens if the mtrr vendor is Intel.  We would 
need to disable that test some other way.

    J
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