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
> > + *
> > + * 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
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