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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: Wed, 20 May 2009 10:01:45 +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>
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On 05/19/09 17:24, Ingo Molnar wrote:
the Xen hypervisor can simply repeat all requests (i.e. not care at
all about the fact that a guest does these modifications on all CPUs
it sees), or realize that the modification has already been done and
skip it.

Could be done, yes. It still feels wrong that wrmsr(mtrr) works slightly different on xen and on native. And it wouldn't work on existing Xen deployments as the Xen hypervisor doesn't support that today.

Yeah, the third one is to not touch MTRRs after bootup and use PAT.

That's a really old CPU, but even Coppermine has PAT support in the
CPU. You need to go back to things like P5 200 MHz CPUs to find
PAT-less CPUs.

Linux shouln't say "PAT not supported by CPU." then.

Also it doesn't make sense to me to handle things differently on native and xen. While it might make sense to deprecate mtrrs in favor of PAT (don't know enougth about all the different cpus in the wild to justify that) I don't think it makes sense to do that for xen only. Native should declare mtrrs obsolete as well.

cheers,
  Gerd


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