Question about ARM LPAE

Adam Lackorzynski adam at os.inf.tu-dresden.de
Tue Apr 5 01:01:04 CEST 2016


On Sun Apr 03, 2016 at 18:25:31 +0800, li94575 wrote:
> Hi,
> >> >> At 2016-03-25 06:41:45, "Adam Lackorzynski" <adam at os.inf.tu-dresden.de> wrote:
> >> >> >
> >> >> >With the current 32bit setup the second part won't be accessible as it
> >> >> >goes beyond the 4GB space. Making this memory usable on 32bit could be
> >> >> >done with some (very hacky) changes but commonly thoughts are that such
> >> >> >an effort would be better spent for 64bit that does not have any such
> >> >> >limitations.
> >>
> >> Could you tell me where I need to change? I have modified the memory initialization in sigma0, and
> >> addeda new memory region (from 0x100000000 to 0x13fffffff) on the avl tree. However, it does
> >> not seem to work, because the page fault address is 32 bit (fpage also cann't hold 64-bit address),
> >> I wonder if I revise the wrong place.
> >
> >The size of an fpage is one thing to solve but far not the only one.
> >Maybe an fpage can be changed to describe larger addresses (with other
> >trade-offs) or sigma0 could use a different way/protocol to talk to the
> >kernel. With that one could get to the memory above 4G through sigma0
> >(the sigma0 protocol for clients would probably also need some review
> >regarding larger addresses). Would that be enough for your use case?
> >
> 
> I do not want to extend all of the physical address to 64 bit, including the fpage, this will involve too many places. So, we come up with a way to avoid using the 64-bit physical address, using 32-bit address (0x40000000 ~ 0x7fffffff)  to replace the 33-bit physical address (0x100000000 ~ 0x13fffffff). When filling in the page table entry, we make judgments and fill the actual 33-bit physical address. Then, the address translation in MMU would be no problem, the kernel memory area(16MB) is still in the front 1G address.  Is there a problem with this method? We test it in the kernel, it seems to work well, but processes in user space cannot use the address(0x40000000 ~ 0x7fffffff).

Ok, looks like a good idea to me and I think it should work.
What happens when user programs access the second GB?
And which programs, moe, or later ones?



Adam
-- 
Adam                 adam at os.inf.tu-dresden.de
  Lackorzynski         http://os.inf.tu-dresden.de/~adam/



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