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[Xen-devel] Re: [PATCH 12/12] xen/mtrr: Add mtrr_if support for Xen mtrr

To: mingo@xxxxxxxxxx, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 12/12] xen/mtrr: Add mtrr_if support for Xen mtrr
From: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
Date: Tue, 28 Sep 2010 15:19:28 +0200
Cc: x86@xxxxxxxxxx, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx
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>* stefano.stabellini@xxxxxxxxxxxxx <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> From: Stephen Tweedie <sct@xxxxxxxxxx>
>> Add a Xen mtrr type, and reorganise mtrr initialisation slightly to
>> allow the mtrr driver to set up num_var_ranges (Xen needs to do this by
>> querying the hypervisor itself.)
>> [ Impact: add basic MTRR support ]
>> Signed-off-by: Stephen Tweedie <sct@xxxxxxxxxx>
>> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>> ---
>> arch/x86/kernel/cpu/mtrr/Makefile | 2 +-
>> arch/x86/kernel/cpu/mtrr/main.c | 3 +
>> arch/x86/kernel/cpu/mtrr/mtrr.h | 7 ++
>> arch/x86/kernel/cpu/mtrr/xen.c | 110 +++++++++++++++++++++++++++++++++++++
> 4 files changed, 121 insertions(+), 1 deletions(-)
> create mode 100644 arch/x86/kernel/cpu/mtrr/xen.c
>Still NAK, for the very same reasons as we NAK-ed it the previous time: 
>/proc/mtrr is a problematic and complicated legacy interface that should 
>die. Any modern X server will do the right thing via PAT.
>Also, please get the Ack of at least one x86 maintainer for x86 patches.

I can't find the MTRR API to be officially deprecated with any schedule for 
removal (at least i couldn't find it in feature-removal-schedule.txt).
KVM has had patches for MTRR as well, if it's so deprecated .. why has it ? .. 
legacy support for a not officially deprecated API perhaps ? To support CPU's 
that support MTRR but not PAT ?

If you think the MTRR interface should die, and should die fast, why hasn't it 
been added to feature-removal-schedule.txt for a complete removal in a not so 
distant future, since this is the only way to force all users to switch to PAT.

Perhaps it would be fair to ask for a working PAT implementation besides MTRR, 
but since it is not officially deprecated completely objecting a MTRR one just 
for Xen seems a bit unfair to me.


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