Questions on Hugetlb for bookE and PHYS_64BIT

[Becky Bruce]

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

> On 06/15/2012 09:43 AM, telenn barz wrote:
>> Hi all,
>> 
>> CONFIG_PHYS_64BIT enables kernel support for larger than 32-bit physical
>> 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) ?
> 
> Yes.
> 
>> The Hugetlb patch for BookE
>> (https://lists.ozlabs.org/pipermail/linuxppc-dev/2011-June/091315.html)
>> seems to be surbordinated to CONFIG_PHYS_64BIT. Is there any good reason
>> why it is not supported when CONFIG_PHYS_64BIT is disabled ?
> 
> 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. 

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

> 
> -Scott
> 
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