x86/mm: Add ioremap_uc() helper to map memory uncacheable (not UC-)

ioremap_nocache() currently uses UC- by default. Our goal is to
eventually make UC the default. Linux maps UC- to PCD=1, PWT=0
page attributes on non-PAT systems. Linux maps UC to PCD=1,
PWT=1 page attributes on non-PAT systems. On non-PAT and PAT
systems a WC MTRR has different effects on pages with either of
these attributes. In order to help with a smooth transition its
best to enable use of UC (PCD,1, PWT=1) on a region as that
ensures a WC MTRR will have no effect on a region, this however
requires us to have an way to declare a region as UC and we
currently do not have a way to do this.

  WC MTRR on non-PAT system with PCD=1, PWT=0 (UC-) yields WC.
  WC MTRR on non-PAT system with PCD=1, PWT=1 (UC)  yields UC.

  WC MTRR on PAT system with PCD=1, PWT=0 (UC-) yields WC.
  WC MTRR on PAT system with PCD=1, PWT=1 (UC)  yields UC.

A flip of the default ioremap_nocache() behaviour from UC- to UC
can therefore regress a memory region from effective memory type
WC to UC if MTRRs are used. Use of MTRRs should be phased out
and in the best case only arch_phys_wc_add() use will remain,
even if this happens arch_phys_wc_add() will have an effect on
non-PAT systems and changes to default ioremap_nocache()
behaviour could regress drivers.

Now, ideally we'd use ioremap_nocache() on the regions in which
we'd need uncachable memory types and avoid any MTRRs on those
regions. There are however some restrictions on MTRRs use, such
as the requirement of having the base and size of variable sized
MTRRs to be powers of two, which could mean having to use a WC
MTRR over a large area which includes a region in which
write-combining effects are undesirable.

Add ioremap_uc() to help with the both phasing out of MTRR use
and also provide a way to blacklist small WC undesirable regions
in devices with mixed regions which are size-implicated to use
large WC MTRRs. Use of ioremap_uc() helps phase out MTRR use by
avoiding regressions with an eventual flip of default behaviour
or ioremap_nocache() from UC- to UC.

Drivers working with WC MTRRs can use the below table to review
and consider the use of ioremap*() and similar helpers to ensure
appropriate behaviour long term even if default
ioremap_nocache() behaviour changes from UC- to UC.

Although ioremap_uc() is being added we leave set_memory_uc() to
use UC- as only initial memory type setup is required to be able
to accommodate existing device drivers and phase out MTRR use.
It should also be clarified that set_memory_uc() cannot be used
with IO memory, even though its use will not return any errors,
it really has no effect.

  ----------------------------------------------------------------------
  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
  ----------------------------------------------------------------------

Signed-off-by: Luis R. Rodriguez <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Acked-by: H. Peter Anvin <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Antonino Daplas <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Jean-Christophe Plagniol-Villard <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Mike Travis <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Suresh Siddha <[email protected]>
Cc: Thierry Reding <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tomi Valkeinen <[email protected]>
Cc: Toshi Kani <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

0 files changed, 0 insertions, 0 deletions