Re: IP27: CONFIG_TRANSPARENT_HUGEPAGE triggers bus errors

[Joshua Kinard <kumba@gentoo.org>]

On 11/05/2014 11:09, Ralf Baechle wrote:
> On Mon, Nov 03, 2014 at 05:23:29PM -0800, David Daney wrote:
> 
>> I haven't checked, but there may be workarounds required in the TLB
>> management code that are not in place for the huge page case.  When the huge
>> TLB code was developed, we didn't do any testing on R10K.  Somebody should
>> dump the exception handlers and carefully look at the rest of the huge TLB
>> management code, and check to see that any required workarounds are in
>> place.
> 
> Joshua, if you happen to have R10000 errata sheets around, maybe you could
> check if there's anything suspicious?  Off the top of my head I don't recall
> any R10000 TLB erratas but the R10000 had plenty of erratas due to it's - by
> the standards of the time - high complexity.
> 
>   Ralf

All I have are errata sheets for Rev 2.3, 2.4, and 2.5 of the R10K.  Nothing
specific on the R12K, and nil for the R14K/R16K.

That said, poking through other areas of the R10K/R12K User Manual, there are
paragraphs titled "Errata" and regarding the PageMask register or TLB, they
state this:

Page 41
The calculated address is translated from a 44-bit virtual address into a
40-bit physical address using a translation-lookaside buffer. The TLB contains
64 entries, each of which can translate two pages. Each entry can select a page
size ranging from 4 Kbytes to 16 Mbytes, inclusive, in __powers__ of 4, as
shown in Figure 1-6.

Page 316:
Translated virtual addresses retrieve data in blocks, which are called pages.
In the R10000 processor, the size of each page may be selected from a range
that runs from 4 Kbytes to 16 Mbytes inclusive, __in_powers_of_4__ (that is, 4
Kbytes, 16 Kbytes, 64 Kbytes, etc.).

So my guess is unless hugepages can happen in powers of 4, they're not
compatible w/ the R10K-series (and likely not the R5K/RM7K, either, since they
all have the same 24:13 bits in the PageMask register).  It seems the logical
choice would be to remove 'select CPU_SUPPORTS_HUGEPAGES' from CPU_R5000,
CPU_NEVADA, CPU_R10000, and CPU_RM7000 in arch/mips/Kconfig.

-- 
Joshua Kinard
Gentoo/MIPS
kumba@gentoo.org
4096R/D25D95E3 2011-03-28

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