Re: MTRR use in drivers

From: H. Peter Anvin
Date: Sun Jun 23 2013 - 20:31:58 EST

The aliasing doesn't matter for Linux because we map the high and low half the same.

Henrique de Moraes Holschuh <hmh@xxxxxxxxxx> wrote:

>On Sun, 23 Jun 2013, H. Peter Anvin wrote:
>> On 06/23/2013 02:56 PM, Henrique de Moraes Holschuh wrote:
>> >
>> > And as far as I could find from Intel's not-that-complete public
>> > "specification updates", we are applying the errata workaround to a
>few more
>> > processors than strictly required, but since I have no idea how to
>write a
>> > test case, I can't whitelist the 3rd-gen Pentium M on my T43, nor
>can I get
>> > ThinkPad owners to test it for us on 1st and 2nd-gen Pentium M and
>> > back.
>> Which specific erratum are you referring to, here? The "WC becomes
>> erratum? I don't think there is a sane testcase for it since it
>needs a
>> very complicated setup to trigger.
>There are at least two different nasty PAT issues that are not always
>critical, and one that outright hangs the processor (if the unsupported
>aliasing of WB with UC/WC happens).
>Interestingly enough, most of the P4-Xeons and P4 do not appear to have
>"WC becomes UC" errata.
>However, LOTS of P4, M-P4, Xeon PIII, Xeon, and Pentium M have a bug
>the four highest entries in the PAT table are inactive (aliased to the
>lowest entries) in mode B (PSE) and mode C (PAE) for 4k pages. They
>fine for large pages.
>Also, lots of them can hang if you ever alias WB with UC or WC (which
>apparently an unsupported configuration anyway, or so it says in the
>There are other weird aliasing nasties, such as one where you get
>corruption if you alias WB data with code (being accessed as UC or WC)
>the same cacheline, and some stuff such as weirdness should the page
>be on WC memory...
>I can track down most of the CPUIDs involved if you want, but someone
>Intel would be better (I assume they actually have access to the errata
>documentation in some less idiotic way than reading a ton of badly
>PDFs that take forever to find in their site).

Sent from my mobile phone. Please excuse brevity and lack of formatting.
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