Re: [RFC PATCH -tip] x86/pat: fix pat_x_mtrr_type to its essentials

[Xiaotian Feng]

On 12/01/2009 07:21 AM, Suresh Siddha wrote:
> Xiaotian,
>
> We look through the mtrr's only when the incoming request type is WB
> (which is mostly the case for legacy /dev/mem mmap mapping). And only in
> this scenario we use more conservative UC- instead of WC, if mtrr says
> WC.
>
> This will ensure the scenarios like ioremap() (which defaults to UC-) by
> legacy drivers, before and after the user level mmap are consistent and
> don't fail because of inconsistent mappings etc.
>
> There is no reason why we need to use WC when especially the incoming
> request was not strict enough and specified WB.
>
> Also BTW,  your current effective return values mentioned in the below
> table () is wrong.
>
> i.e., for example, when we use UC- for PAT and mtrr has WC, effective
> memory type used by cpu will be WC.
Thanks for the detailed explanation, I'm clear now, thank you very much.
> thanks,
> suresh
>
> On Thu, 2009-11-26 at 04:15 -0800, Xiaotian Feng wrote:
> > Due to the IA-32 SDM vol 3a "Effective Memory Type" tables, the PAT
> > and mtrr relations should be:
> > 	|	UC	UC-	WC	WB	[PAT]
> > --------+--------------------------------------
> > UC	|	UC	UC(UC-)	WC	UC(UC-)
> > WC	|	UC	WC(UC-)	WC	WC(UC-)
> > WB	|	UC	UC(UC-)	WC	WB
> > [MTRR]
> > * In () is the current return value.
> >
> > So, commit b6ff32, Fix pat_x_mtrr_type() to use UC_MINUS when the
> > mtrr type return UC. But it also made mtrr type WC use UC_MINUS.
> > This is not reasonable.
> >
> > Signed-off-by: Xiaotian Feng<dfeng@xxxxxxxxxx>
> > Cc: Thomas Gleixner<tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar<mingo@xxxxxxxxxx>
> > Cc: H. Peter Anvin<hpa@xxxxxxxxx>
> > Cc: Suresh Siddha<suresh.b.siddha@xxxxxxxxx>
> > Cc: Venkatesh Pallipadi<venkatesh.pallipadi@xxxxxxxxx>
> > ---
> >   arch/x86/mm/pat.c |    9 +++++----
> >   1 files changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> > index c37fd51..92e9627 100644
> > --- a/arch/x86/mm/pat.c
> > +++ b/arch/x86/mm/pat.c
> > @@ -228,10 +228,11 @@ static unsigned long pat_x_mtrr_type(u64 start, u64 end, unsigned long req_type)
> >   		u8 mtrr_type;
> >
> >   		mtrr_type = mtrr_type_lookup(start, end);
> > -		if (mtrr_type != MTRR_TYPE_WRBACK)
> > -			return _PAGE_CACHE_UC_MINUS;
> > -
> > -		return _PAGE_CACHE_WB;
> > +		if (mtrr_type == MTRR_TYPE_WRBACK)
> > +			return _PAGE_CACHE_WB;
> > +		else if (mtrr_type == MTRR_TYPE_WRCOMB)
> > +			return _PAGE_CACHE_WC;
> > +		return _PAGE_CACHE_UC_MINUS;
> >   	}
> >
> >   	return req_type;

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