Re: Page sizes supported by RISC-V

Maciej W. Rozycki
2025-01-15T09:03:29Z

On Wed, 15 Jan 2025, 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.
> >
> > Can't we simply make a reasonable guess/assumption just for the purpose
> > of tooling?
>
> That reasonable default exists today and is 4096. Some effort is
> required to change that.

I thought we were talking about the maximum rather than the default. I
don't think changing the default causes any fuss as long as it's between
previously established minimum and maximum.

Maciej