Re: [PATCH v4 1/7] x86, mm, pat: Set WT to PA7 slot of PAT MSR

[Andy Lutomirski]

On Mon, Nov 3, 2014 at 9:47 AM, Toshi Kani <toshi.kani@xxxxxx> wrote:
> On Mon, 2014-11-03 at 18:14 +0100, Thomas Gleixner wrote:
>> On Mon, 27 Oct 2014, Toshi Kani wrote:
>> > +   } else {
>> > +           /*
>> > +            * PAT full support. WT is set to slot 7, which minimizes
>> > +            * the risk of using the PAT bit as slot 3 is UC and is
>> > +            * currently unused. Slot 4 should remain as reserved.
>>
>> This comment makes no sense. What minimizes which risk and what has
>> this to do with slot 3 and slot 4?
>
> This is for precaution.  Since the patch enables the PAT bit the first
> time, it was suggested that we keep slot 4 reserved and set it to WB.
> The PAT bit still has no effect to slot 0/1/2 (WB/WC/UC-) after this
> patch.  Slot 7 is the safest slot since slot 3 (UC) is unused today.
>
> https://lkml.org/lkml/2014/9/4/691
> https://lkml.org/lkml/2014/9/5/394
>

I would clarify the comment, since this really has nothing to do with
slot 3 being unused.  How about:

We put WT in slot 7 to improve robustness in the presence of errata
that might cause the high PAT bit to be ignored.  This way a buggy
slot 7 access will hit slot 3, and slot 3 is UC, so at worst we lose
performance without causing a correctness issue.  Pentium 4 erratum
N46 is an example of such an erratum, although we try not to use PAT
at all on affected CPUs.

--Andy


>> > +            *
>> > +            *  PTE encoding used in Linux:
>> > +            *      PAT
>> > +            *      |PCD
>> > +            *      ||PWT  PAT
>> > +            *      |||    slot
>> > +            *      000    0    WB : _PAGE_CACHE_MODE_WB
>> > +            *      001    1    WC : _PAGE_CACHE_MODE_WC
>> > +            *      010    2    UC-: _PAGE_CACHE_MODE_UC_MINUS
>> > +            *      011    3    UC : _PAGE_CACHE_MODE_UC
>> > +            *      100    4    <reserved>
>> > +            *      101    5    <reserved>
>> > +            *      110    6    <reserved>
>>
>> Well, they are still mapped to WB/WC/UC_MINUS ....
>
> Right, the reserved slots are also initialized with their safe values.
> However, the macros _PAGE_CACHE_MODE_XXX only refer to the slots
> specified above.
>
>> > +            *      111    7    WT : _PAGE_CACHE_MODE_WT
>> > +            */
>> > +           pat = PAT(0, WB) | PAT(1, WC) | PAT(2, UC_MINUS) | PAT(3, UC) |
>> > +                 PAT(4, WB) | PAT(5, WC) | PAT(6, UC_MINUS) | PAT(7, WT);
>> > +   }
>>
>> Thanks,
>
> Thanks for the review!
> -Toshi
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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