Re: [PATCH 0/8] x86, acpi: Move acpi_initrd_override() earlier.

[Tejun Heo <tj@kernel.org>]

On Fri, Aug 23, 2013 at 10:35:07AM -0400, Tejun Heo wrote:
> Yeah, it's true that MTRRs are nasty.  On the other hand, we've been
> doing that for over a decade and are still doing it anyway if I'm not
> mistaken.  It probably isn't a big difference but it's still a bit sad
> that this is likely causing small performance regression out in the
> wild.

Just went over the processor manual and it doesn't seem like doing the
above would be a good idea.


  System Programming Guide, Part 1

  11.11.9 Large Page Size Considerations

 ... 
 Because the memory type for a large page is cached in the TLB, the
 processor can behave in an undefined manner if a large page is mapped
 to a region of memory that MTRRs have mapped with multiple memory
 types.
 ...
 If a large page maps to a region of memory containing different
 MTRR-defined memory types, the PCD and PWT flags in the page-table
 entry should be set for the most conservative memory type for that
 range. For example, a large page used for memory mapped I/O and
 regular memory 11-48 Vol. 3A MEMORY CACHE CONTROL
 ...

 The Pentium 4, Intel Xeon, and P6 family processors provide special
 support for the physical memory range from 0 to 4 MBytes,
 ...
 Here, the processor maps the memory range as multiple 4-KByte pages
 within the TLB. This operation insures correct behavior at the cost
 of performance. To avoid this performance penalty, operating-system
 software should reserve the large page option for regions of memory
 at addresses greater than or equal to 4 MBytes.

So, yeah, the current behavior seems like the right thing to do.

Thanks.

-- 
tejun

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