Re: [PATCH 3/5] MIPS: Set page size to 16KB for malta SMP defconfigs

[Ralf Baechle <ralf@linux-mips.org>]

On Mon, Mar 03, 2014 at 02:29:21PM +0000, Paul Burton wrote:

> On Mon, Mar 03, 2014 at 02:06:18PM +0000, Markos Chandras wrote:
> > On 02/21/2014 05:38 PM, Ralf Baechle wrote:
> > >On Thu, Feb 20, 2014 at 02:00:26PM +0000, Markos Chandras wrote:
> > >
> > >>From: Paul Burton <paul.burton@imgtec.com>
> > >>
> > >>For Malta defconfigs which may run on an SMP configuration without
> > >>hardware cache anti-aliasing, a 16KB page size is a safer default.
> > >>Most notably at the moment it will avoid cache aliasing issues for
> > >>multicore proAptiv systems.
> > >
> > >You're aware that this may cause binary compatibility issues with old
> > >userland?  So far the defaults were chosen to maximise compatibility
> > >over performance.
> > >
> > >   Ralf
> > >
> > Hi Ralf,
> > 
> > Are you referring to programs hard coding the page size to 4k instead of
> > using the getpagesize()? Well yes this could be a problem. But is that a
> > real problem? We are changing the default value so whoever has such an old
> > userland can easily switch to the 4k page size. It may also be a good
> > opportunity to expose such application and get the fixed properly :) But if
> > that's not acceptable, we can drop the patch. Paul what do you think?
> > 
> > -- 
> > markos
> 
> I think the potential backwards compatibility issue is probably not a
> huge issue for Malta, but that it would make sense to dig deeper into
> why proAptiv SMP systems don't run correctly with 4KB pages. You don't
> actually have to look very far to find userland which depends upon 4KB
> pages (*cough* android *cough*). I consider this patch an acceptable
> workaround for the proAptiv problem on Malta, but I don't know enough
> at the moment to say whether there's a proper fix.

The issue I was refering to is that certain older binutils are creating
ELF segments of insufficient alignment for large page sizes to work.

Hardcoded page size is another problem - one should think that after
iterating through every power of two starting with like 512 bytes
programmers have sprung a clue.

  Ralf