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

[Tobias Platen]

On 19.07.19 23:01, Luke Kenneth Casson Leighton wrote:
> 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.
>
> l.
>
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Today I have attempted to implement the 48-bit version of the PTW, I 
reran the tests and got no errors.

But it seems there is no test for the TLB as a whole yet, so I don't 
know whether the returned address is correct.

Tobias