Re: Questions on Hugetlb for bookE and PHYS_64BIT

On Jun 15, 2012, at 1:11 PM, Scott Wood wrote:

On 06/15/2012 09:43 AM, telenn barz wrote:

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Hi all,
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CONFIG_PHYS_64BIT enables kernel support for larger than 32-bit =

physical

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addresses. Is it this configuration option we have to enable for the
support of 36-bit real memory (as are capable the Freescale e500v2 or
e500mc cores family) ?

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Yes.
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The Hugetlb patch for BookE
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(https://lists.ozlabs.org/pipermail/linuxppc-dev/2011-June/091315.html)

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seems to be surbordinated to CONFIG_PHYS_64BIT. Is there any good =

reason

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why it is not supported when CONFIG_PHYS_64BIT is disabled ?

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Because it would be extra work to support that configuration, and
nobody's felt enough of a need for it to put in that work.

Most of the use-cases we had demanding hugetlb were *also* cases that =
had large amounts of memory, so this is the model we adopted.  Plus, we =
need some of the code from the 36-bit implementation, including the part =
that causes the lowest level of the page table to widen to 64 bits; =
additionally we did not measure much, if any, of a perf hit when =
enabling PHYS_64BIT.=20

Obviously, it could be made to work without it, although there are real =
code changes required. But unless you have a case where you have a =
noticeable performance drop from enabling CONFIG_PHYS_64BIT, it's not =
worth the effort.

-Becky

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-Scott
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