On Mon, 3 Nov 2014, Andy Lutomirski wrote:
> 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.
Indeed. That makes a lot more sense.
> >> > + *
> >> > + * 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.
Then the table should reflect this, i.e.: reserved, but mapped to XX
And a comment below that explaining WHY we map the reserved slots.
Thanks,
tglx
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