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

Ingo Molnar
2015-05-20T15:01:23Z

* Toshi Kani <toshi.kani@hp.com> wrote:

> On Wed, 2015-05-20 at 13:55 +0200, Ingo Molnar wrote:
> > * Borislav Petkov <bp@alien8.de> wrote:
> >
> > > --- a/arch/x86/mm/pgtable.c
> > > +++ b/arch/x86/mm/pgtable.c
> > > @@ -566,19 +566,28 @@ 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 any of the following conditions are met:
> > > + *
> > > + * - 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.
> > > + *
> > > + * Callers should try to decrease page size (1GB -> 2MB -> 4K) if the bigger
> > > + * page mapping attempt fails.
> >
> > This comment should explain why it's ok in the WB case.
> >
> > Also, the phrase 'the memory type of the range' is ambiguous: it might
> > mean the partial MTRR's, or the memory type specified via PAT by the
> > huge-pmd entry.
>
> Agreed. How about this sentence?
>
> - MTRRs are enabled and the corresponding MTRR memory type is WB, which
> has no effect to the requested PAT memory type.

s/effect to/effect on

sounds good otherwise!

Btw., if WB MTRR entries can never have an effect on Linux PAT
specified attributes, why do we allow them to be created? I don't
think we ever call into real mode for this to matter?

Thanks,

Ingo