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

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Message-ID: <20150520150114.GA19161@gmail.com>
Date:	Wed, 20 May 2015 17:01:14 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Toshi Kani <toshi.kani@...com>
Cc:	Borislav Petkov <bp@...en8.de>, akpm@...ux-foundation.org,
	hpa@...or.com, tglx@...utronix.de, mingo@...hat.com,
	linux-mm@...ck.org, x86@...nel.org, linux-kernel@...r.kernel.org,
	dave.hansen@...el.com, Elliott@...com, pebolle@...cali.nl,
	mcgrof@...e.com
Subject: Re: [PATCH v5 6/6] mtrr, mm, x86: Enhance MTRR checks for KVA huge
 page mapping


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

> On Wed, 2015-05-20 at 13:55 +0200, Ingo Molnar wrote:
> > * Borislav Petkov <bp@...en8.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
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