[lowrisc-dev] Re: [sw-dev] Is a de facto standard memory map helpful or harmful?

Arnd Bergmann arnd at arndb.de
Thu Jul 14 13:55:37 BST 2016


On Thursday, July 14, 2016 5:52:28 AM CEST krste at berkeley.edu wrote:

> | Unfortunately, the memory map described on page 12 of that document
> | makes the same mistake as some ARM64 chips and makes the RAM
> | location extremely sparse, with the first 14GB starting close to
> | zero, but all RAM beyond that starting at 0x1_0000_0000_0000
> | (256TB), which I guess requires using an extra level of page tables
> | for the kind of linear mapping that Linux has. It's probably too
> | late to change that, but I'd suggest that future implementations
> | do it differently.
> 
> Platforms only need populate a single linear RAM address region.  The
> memory map describes the options where this could be put.
> 
> The reason we, and many others, adopt this style of layout is to
> reduce hardware costs for smaller implementations which do not need to
> provide all physical address bits and to support various DIMM sizes
> without adding complexity/delay to memory accesses and coherence
> systems.

Ok, that makes sense, and only leaves the problem of relocating
the kernel at boot time to the correct physical address.

	Arnd



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