[libre-riscv-dev] Just reading the spec again

Luke Kenneth Casson Leighton lkcl at lkcl.net
Fri Jul 19 22:01:27 BST 2019

On Fri, Jul 19, 2019 at 9:59 PM Jacob Lifshay <programmerjake at gmail.com> wrote:
> On Fri, Jul 19, 2019, 12:30 Luke Kenneth Casson Leighton <lkcl at lkcl.net>
> wrote:
> > Btw as we may be the first team to do such large VM TLB pages, it would
> > probably be a good idea to make it runtime selectable, 3 or 4 levels.
> >
> The spec requires support for the 39-bit virtual address mode. I think
> supporting both the 39-bit and 48-bit versions is a good idea, since 39-bit
> only supports a 512GiB address space and it's relatively trivial to imagine
> a scenario where multiple SoCs are wired together to make a coherent shared
> memory cluster (over OmniXtend) and/or to have a 1TiB memory mapped
> database, 57-bit mode is unnecessarily big and not officially defined yet.
> btw, the 3-level version has 12+3*9=39 virtual address bits, the 4-level
> version has 12+4*9=48 virtual address bits, the 5-level version has
> 12+5*9=57 virtual address bits (currently reserved by the spec).

 okaaay, apologies, tobias: i meant the 48-bit version.  4 levels, not 5.


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