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SubjectRe: [GIT PULL] xen /proc/mtrr implementation
On Fri, 15 May 2009 16:49:12 -0700
Jeremy Fitzhardinge <> wrote:

> Eric W. Biederman wrote:
> >> /proc/mtrr is in wide use today. It may be planned for
> >> obsolescence, but there's no way you can claim its obsolete today
> >> (my completely up-to-date F10 X server is using it, for example).
> >> We don't break oldish usermode ABIs in new kernels.
> >>
> >
> > Sure it is. There is a better newer replacement. It is taking a
> > while to get userspace transitioned but that is different.
> > Honestly I am puzzled why that it but whatever.
> >
> There's no mention in feature-removal-schedule.txt.
> >> Besides, the MTRR code is also a kernel-internal API, used by DRM
> >> and other drivers to configure the system MTRR state. Those
> >> drivers will either perform badly or outright fail if they can't
> >> set the appropriate cachability properties. That is not obsolete
> >> in any way.
> >
> > There are about 5 of them so let's fix them.
> >
> Well, I count at least 30+, but anyway.
> > With PAT we are in a much better position both for portability and
> > for flexibility.
> >
> PAT is relatively recent, and even more recently bug-free. There are
> many people with processors which can't or won't do PAT; what's the
> plan to support them? Just hit them with a performance regression?
> Or wrap MTRR in some other API?
> > Is it possible to fix PAT and get that working first. That is
> > very definitely the preferend API.
> >
> Sure, when available. We're sorting out the details for Xen, but
> even then it may not be available, either because we're running on an
> old version of Xen, or because some other guest is using PAT
> differently.
> But I honestly don't understand the hostility towards 120 lines of
> code to make an interface (albeit legacy/deprecated/whatever) behave
> in an expected way.

FWIW I think supporting the MTRR API in Xen makes sense. There's a lot
of old code out there that wants it; would be nice if it mostly worked,
especially at such a minimal cost. It's taken awhile to get PAT going
(and there are still issues here and there) so having the MTRR stuffa
available is awfully nice.

Jesse Barnes, Intel Open Source Technology Center

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