Re: [PATCH v3 2/2] ARM : change fixmap mapping region to support 32 CPUs

[Kees Cook]

On Fri, May 30, 2014 at 12:25 PM, Nicolas Pitre
<nicolas.pitre@xxxxxxxxxx> wrote:
> On Fri, 30 May 2014, Rob Herring wrote:
>
>> There's work in flight to support early_ioremap, early console, and RO
>> text patching which all use the fixmap region.
>>
>> There's a couple of options to solve this:
>>
>> - Only support up to 16 cpus. It could be anywhere between 17-31, but
>> that seems somewhat unlikely. Are we really ever going to see 32-bit
>> 32 core systems?
>
> I wouldn't rule that out.  I've seen 16-core ARM chips in 2008 (although
> they didn't go into production).  Silly limitations like that always
> come back to bite you.  And we have better alternatives.
>
>> - Reduce KM_TYPE_NR from 16 to 15. Based on the comment for it, we
>> probably don't want to do that. Is increasing it to the default of 20
>> worthwhile? Some of the options here would allow doing that.
>> - Add 0xffe00000-0xfff00000 to the fixmap region. This would make
>> fixmap span 2 PMDs with the top PMD having a mixture of uses like we
>> had before.
>
> That would be my preferred approach.  Note here it could be
> 0xffe00000-0xfffe0000 to include the whole of the previous fixmap area
> curently unused.
>
>> - push the PCI i/o space down to 0xfec00000 and make fixmap 4MB. This
>> is a cleaner solution as the 2 PMDs are only used for fixmap. This may
>> require some static mapping adjustments on some platforms.
>
> No need.  With the latest changes, the fixmap area is between 0xffc00000
> and 0xffe00000 (there is apparently a mistake in
> Documentation/arm/memory.txt).  So currently 0xff000000-0xffc00000 is
> free, which makes the fixmap area far away from the PCI i/o area with
> plenti of space in between.

So, it seems there is something wrong with this patch series. I had to
revert "ARM: 8031/2: change fixmap mapping region to support 32 CPUs"
to make other fixmap changes work correctly. I think this is due to
the non-highmem config case moving the fixmap to a location where
there is to page table entry...

-Kees

>
>> - Same as previous option, but convert the PCI i/o space to fixmap
>> entries. We don't really need all 2MB for PCI.
>
> See above.
>
>> Also, there is an error in the documentation below:
>>
>> >
>> > Signed-off-by: Liu Hua <sdu.liu@xxxxxxxxxx>
>> > ---
>> >  Documentation/arm/memory.txt   |  2 +-
>
> Yep, good that you spotted it as well.  I failed to catch it during my
> review so I'll send a patch.
>
>
> Nicolas
>
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-- 
Kees Cook
Chrome OS Security
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