Re: [PATCH v4 2/6] x86: document WC MTRR effects on PAT / non-PAT pages

Luis R. Rodriguez
2015-05-05T00:45:13Z

On Thu, Apr 30, 2015 at 03:01:12PM -0700, Randy Dunlap wrote:
> On 04/29/15 14:44, Luis R. Rodriguez wrote:
> > From: "Luis R. Rodriguez" <mcgrof@suse.com>
> > diff --git a/Documentation/x86/pat.txt b/Documentation/x86/pat.txt
> > index cf08c9f..7e183e3 100644
> > --- a/Documentation/x86/pat.txt
> > +++ b/Documentation/x86/pat.txt
> > @@ -102,7 +104,43 @@ wants to export a RAM region, it has to do set_memory_uc() or set_memory_wc()
> > as step 0 above and also track the usage of those pages and use set_memory_wb()
> > before the page is freed to free pool.
> >
> > -
> > +MTRR effects on PAT / non-PAT systems
> > +-------------------------------------
> > +
> > +The following table provides the effects of using write-combining MTRRs when
> > +using ioremap*() calls on x86 for both non-PAT and PAT systems. Ideally
> > +mtrr_add() usage will be phased in favor of arch_phys_wc_add() which will
> > +be a no-op on PAT enabled systems. The region over which a arch_phys_wc_add()
> > +is made should already have be ioremap'd with write-combining page attributes
> > +or PAT entries, this can be done by using ioremap_wc() / or respective helpers.
> > +Devices which combine areas of IO memory desired to remain uncachable with
>
> I would spell it uncacheable. In kernel Documentation/, grep uncacheable finds
> 14 hits vs. 6 hits for uncachable. No big deal.
>
> > +areas where write-combining is desirable and are restricted by the size
> > +requirements of MTRRs should consider splitting up their IO memory space
> > +cleanly with ioremap_uc() and ioremap_wc() followed by an arch_phys_wc_add()
> > +encompassing both regions. Such use is nevertheless heavily discouraged as
> > +the effective memory type is considered implementation defined. This strategy
> > +should only be used as last resort on devices with size-contrained regions
>
> size-constrained
>
> > +where otherwise MTRR write-combining would not be effective.
> > +
> > +Note that you cannot use set_memory_wc() to override / whitelist IO remapped
> > +memory space mapped with ioremap*() calls, set_memory_wc() can only be used
> > +on RAM.
> > +
> > +----------------------------------------------------------------------
> > +MTRR Non-PAT PAT Linux ioremap value Effective memory type
> > +----------------------------------------------------------------------
> > + Non-PAT | PAT
> > + PAT
> > + |PCD
> > + ||PWT
> > + |||
> > +WC 000 WB _PAGE_CACHE_MODE_WB WC | WC
> > +WC 001 WC _PAGE_CACHE_MODE_WC WC* | WC
> > +WC 010 UC- _PAGE_CACHE_MODE_UC_MINUS WC* | WC
> > +WC 011 UC _PAGE_CACHE_MODE_UC UC | UC
> > +----------------------------------------------------------------------
> > +
> > +(*) denotes implementation defined and is discouraged
> >
> > Notes:
> >
> > diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c
> > index ea5f363..12abdbe 100644
> > --- a/arch/x86/kernel/cpu/mtrr/main.c
> > +++ b/arch/x86/kernel/cpu/mtrr/main.c
> > @@ -538,6 +538,9 @@ EXPORT_SYMBOL(mtrr_del);
> > * attempts to add a WC MTRR covering size bytes starting at base and
> > * logs an error if this fails.
> > *
> > + * The caller should expect to need to provide a power of two size on an
>
> * The called should provide a power of two size on an equivalent
> * power of two boundary.
>

Thanks since Boris took this already I'll let him amend unless he wishes for
me to send a new version.

Luis