Questions on Hugetlb for bookE and PHYS_64BIT

Questions on Hugetlb for bookE and PHYS_64BIT

[telenn barz]

[-- Attachment #1: Type: text/plain, Size: 496 bytes --]

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) ?

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 ?

Thanks,
Telenn

[-- Attachment #2: Type: text/html, Size: 609 bytes --]

Re: Questions on Hugetlb for bookE and PHYS_64BIT

[Scott Wood]

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.

-Scott

Re: 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,
>>=20
>> 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) ?
>=20
> Yes.
>=20
>> 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 ?
>=20
> 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

>=20
> -Scott
>=20
> _______________________________________________
> Linuxppc-dev mailing list
> Linuxppc-dev@lists.ozlabs.org
> https://lists.ozlabs.org/listinfo/linuxppc-dev