On Thu, 2014-09-04 at 14:31 -0600, Toshi Kani wrote:
> On Thu, 2014-09-04 at 17:11 -0300, Henrique de Moraes Holschuh wrote:
> > On Thu, 04 Sep 2014, Toshi Kani wrote:
> > > This patch sets WT to the PA4 slot in the PAT MSR when the processor
> > > is not affected by the PAT errata. The upper 4 slots of the PAT MSR
> > > are continued to be unused on the following Intel processors.
> > >
> > > errata cpuid
> > > --------------------------------------
> > > Pentium 2, A52 family 0x6, model 0x5
> > > Pentium 3, E27 family 0x6, model 0x7
> > > Pentium M, Y26 family 0x6, model 0x9
> > > Pentium 4, N46 family 0xf, model 0x0
> > >
> > > For these affected processors, _PAGE_CACHE_MODE_WT is redirected to UC-
> > > per the default setup in __cachemode2pte_tbl[].
> >
> > There are at least two PAT errata. The blacklist is in
> > arch/x86/kernel/cpu/intel.c:
> >
> > if (c->x86 == 6 && c->x86_model < 15)
> > clear_cpu_cap(c, X86_FEATURE_PAT);
> >
> > It covers model 13, which is not in your blacklist.
> >
> > It *is* possible that PAT would work on model 13, as I don't think it has
> > any PAT errata listed and it was blacklisted "just in case" (from memory. I
> > did not re-check), but this is untested, and unwise to enable on an aging
> > platform.
> >
> > I am worried of uncharted territory, here. I'd actually advocate for not
> > enabling the upper four PAT entries on IA-32 at all, unless Windows 9X / XP
> > is using them as well. Is this a real concern, or am I being overly
> > cautious?
>
> The blacklist you pointed out covers a different PAT errata, and is
> still effective after this change. pat_init() will call pat_disable()
> and the PAT will continue to be disabled on these processors. There is
> no change for them.
>
> My blacklist covers the PAT errata that makes the upper four bit
> unusable when the PAT is enabled.
I checked more carefully, and it turns out that the processors that have
the WC bug with PAT/MTRR also have the upper four bit bug in PAT as
well. The updated blacklist is:
errata cpuid
--------------------------------------
Pentium 2, A52 family 0x6, model 0x5
Pentium 3, E27 family 0x6, model 0x7, 0x8
Pentium 3 Xeon, G26 family 0x6, model 0x7, 0x8, 0xa
Pentium M, Y26 family 0x6, model 0x9
Pentium M 90nm, X9 family 0x6, model 0xd
Pentium 4, N46 family 0xf, model 0x0
So, the check can be the same as cpu/intel.c, except that early Pentium
4 steppings also have the upper four bit bug. I will update the check.
In any case, this check is only meaningful for P4 since the PAT is
disabled for P2/3/M.
Thanks Henrique for pointing this out!
-Toshi
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