Kernel configuration options

[John Richard Moser]

Bernardo Innocenti wrote:
> Tom Sylla wrote:
> 
>> http://openbios.org/viewvc/cpu/x86/pc/olpc/lxmsrs.fth?view=markup&revision=739&root=OpenFirmware
>> has:
>>   msr: 0000.1810 fdfff000.fd000111.  \ Video (write through), fbsize
>>
>> which is setting the framebuffer as write-combining. (the "write
>> through" comment is incorrect)
> 
> This takes care of the physical mapping, but how would userspace
> be able to mmap the framebuffer into virtual memory without
> additional MMU programming?
> 
> I was under the impression that we also need to cover the whole
> region with small 4KB MMU pages.  This degrades performance
> somewhat due to TLB misses when the CPU accesses the framebuffer.
> 

I missed whether or not the Geode actually has 4MiB huge pages, I 
thought someone said it does.  This being the case, why can't you access 
the 16 (or was it 24?) MiB of memory via a handful (about 1/1024th) of 
huge mappings?  Does x86 MMU not allow for huge MMIO?

The Geode GX has 64 TLB entries right?  I don't know how many the Geode 
LX has, or if there's an L2 TLB.  Obviously, though, this would be a 
major performance boon, what with there being (assuming 24MiB of vram) 5 
probably often used mappings instead of 5120 in an often-used set with a 
probably uneven distribution.

> But I must confess I have limited understanding of the Geode
> architecture, so I may be overlooking something.
> 

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