On Fri, Jan 09, 2015 at 10:55:51AM +0000, Arnd Bergmann wrote:
On Friday 09 January 2015 10:33:07 Catalin Marinas wrote:
On Wed, Jan 07, 2015 at 07:48:48PM +0000, Arnd Bergmann wrote:
In other cases that's actually a good thing. One such example is the "Principles of ARM Memory Maps" document that tells hardware implementers to do a rather complex mapping "To support 36-bit x86 PAE compatible operating systems, such as Linux." but makes life much harder in the process than any of the random mappings we have seen in the wild.
Unfortunately, with any significant amount of RAM (say 16GB), this document becomes pretty useless. It basically forces you to have a very sparse physical address map from 0 to over 40-bit. I wouldn't apply the ARM memory maps doc to server systems.
Are you sure? I was under the impression that this document was targetted specifically at servers.
Ah, sorry for the confusion, I haven't read the latest (apparently from 2012) update which covers 44 and 48-bit memory maps.
The only downside is that for more than 32GB of RAM (up to 512GB) it requires a 40-bit memory map. Given the sparseness, we can't use 3-levels of page table with 4KB pages which can only cover 39-bit. Anyway, not a major issue.