Re: [PATCH v5 6/6] mtrr, mm, x86: Enhance MTRR checks for KVA huge page mapping

Borislav Petkov
2015-05-18T19:01:50Z

On Mon, May 18, 2015 at 11:22:39AM -0600, Toshi Kani wrote:
> > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> > index c30f9819786b..f1894daa79ee 100644
> > --- a/arch/x86/mm/pgtable.c
> > +++ b/arch/x86/mm/pgtable.c
> > @@ -566,19 +566,24 @@ void native_set_fixmap(enum fixed_addresses idx, phys_addr_t phys,
> > /**
> > * pud_set_huge - setup kernel PUD mapping
> > *
> > - * MTRR can override PAT memory types with 4KiB granularity. Therefore,
> > - * this function does not set up a huge page when the range is covered
> > - * by a non-WB type of MTRR. MTRR_TYPE_INVALID indicates that MTRR are
> > - * disabled.
> > + * MTRRs can override PAT memory types with 4KiB granularity. Therefore,
> > + * this function sets up a huge page only if all of the following
> > + * conditions are met:
>
> It should be "if any of the following condition is met". Or, does NOT
> setup if all of ...
>
> > + *
> > + * - MTRRs are disabled.
> > + * - The range is mapped uniformly by an MTRR, i.e. the range is
> > + * fully covered by a single MTRR entry or the default type.
> > + * - The MTRR memory type is WB.

Hmm, ok, so this is kinda like "any" but they also depend on each other.
So it is

If
- MTRRs are disabled

or

- MTRRs are enabled and the range is completely covered by a single MTRR

or

- MTRRs are enabled and the range is not completely covered by a
single MTRR but the memory type of the range is WB, even if covered by
multiple MTRRs.

Right?

So tell me this: why do we need to repeat that over those KVA helpers?
It's not like the callers can do anything about it, can they?

So maybe that comment - expanded into more detail - should be over
mtrr_type_lookup() only. That'll be better, methinks.

Hmm.

--
Regards/Gruss,
Boris.

ECO tip #101: Trim your mails when you reply.
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