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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