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

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Message-ID: <4A12B97C.9040706@redhat.com>
Date:	Tue, 19 May 2009 15:51:56 +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 15:31, Ingo Molnar wrote:
> * Gerd Hoffmann<kraxel@...hat.com>  wrote:
>
>> 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.
>
> Applying MTRR changes to only part of the CPUs is utter madness.

Sure.  Do you read what I'm writing?

>> 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.
>
> Why?

See above.  Xen has to apply the changes to all cpus anyway.

>> Oops, the third "proper technical solutions" is missing.
>
> Yeah, the third one is to not touch MTRRs after bootup and use PAT.

Works only in case the CPU has PAT support.

cheers,
   Gerd

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