WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

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

To: Ingo Molnar <mingo@xxxxxxx>
Subject: Re: [Xen-devel] Re: [GIT PULL] xen /proc/mtrr implementation
From: Gerd Hoffmann <kraxel@xxxxxxxxxx>
Date: Tue, 19 May 2009 15:51:56 +0200
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Jesse Barnes <jbarnes@xxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Delivery-date: Tue, 19 May 2009 06:54:09 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <20090519133138.GA8410@xxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <m1iqk1k708.fsf@xxxxxxxxxxxxxxxxx> <4A10EAC4.9070701@xxxxxxxx> <20090518085902.GE10687@xxxxxxx> <4A11A3F8.1010202@xxxxxxxx> <20090519095918.GA11790@xxxxxxx> <4A12A46A02000078000017E1@xxxxxxxxxxxxxxxxxx> <20090519110837.GA10548@xxxxxxx> <4A12A05C.6050004@xxxxxxxxxx> <20090519122623.GD14305@xxxxxxx> <4A12B244.8070301@xxxxxxxxxx> <20090519133138.GA8410@xxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1b3pre) Gecko/20090513 Fedora/3.0-2.3.beta2.fc11 Lightning/1.0pre Thunderbird/3.0b2
On 05/19/09 15:31, Ingo Molnar wrote:
* Gerd Hoffmann<kraxel@xxxxxxxxxx>  wrote:

On 05/19/09 14:26, Ingo Molnar wrote:
* Gerd Hoffmann<kraxel@xxxxxxxxxx>   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


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel

<Prev in Thread] Current Thread [Next in Thread>