Re: Page sizes supported by RISC-V

Jeff Law
2025-01-15T02:00:12Z

On 9/29/24 9:18 AM, Jeff Law wrote:
>
>
> On 9/29/24 9:08 AM, Florian Weimer wrote:
>>
>> I understand that not making a call in the specifications avoids the
>> need to resolve these conflicts.  But if you don't define a maximum page
>> size there, it is effectively specified as 4096 bytes.  This will cause
>> problems if distributions targeting large systems want a larger page
>> size because it better aligns with the kind of hardware they target.
>> It's really bad for cross-distribution binary compatibility, something
>> that's important outside the embedded space and probably necessary for
>> mainstreaming.
>>
>> (Of course you know all this. 8-)
> Yup.  Once it's baked into one significant distro, it's doing to be
> bloody hard to change.  I'm sure we both remember the PPC pagesize stuff
> from a few years back ;(
>
> I strongly suspect the lack of specification here is mean to give
> degrees of freedom to the implementors, but sometimes those writing the
> specs don't really understand the implication of leaving things like
> this unspecified and how much pain it really causes in the end.
So resurrecting this discussion.

Is there any realistic chance of bumping the minimum pagesize for risc-v
at this point? Xu Lu and his team have some compelling data that
indicates moving forward to 64k could be highly profitable. That work
was done on aarch64, but should be generally applicable to riscv as well.

My biggest worry is if we don't tackle this now, then I suspect the
window closes forever and I'd hate for the spacemit chip in the BPI to
mandate something like this going forward.

jeff