Re: [RFC, PATCHv1 00/28] 5-level paging

From: Arnd Bergmann
Date: Fri Dec 09 2016 - 05:27:14 EST

On Friday, December 9, 2016 6:01:30 AM CET Ingo Molnar wrote:
> > - Handle opt-in wider address space for userspace.
> >
> > Not all userspace is ready to handle addresses wider than current
> > 47-bits. At least some JIT compiler make use of upper bits to encode
> > their info.
> >
> > We need to have an interface to opt-in wider addresses from userspace
> > to avoid regressions.
> >
> > For now, I've included testing-only patch which bumps TASK_SIZE to
> > 56-bits. This can be handy for testing to see what breaks if we max-out
> > size of virtual address space.
> So this is just a detail - but it sounds a bit limiting to me to provide an 'opt
> in' flag for something that will work just fine on the vast majority of 64-bit
> software.
> Please make this an opt out compatibility flag instead: similar to how we handle
> address space layout limitations/quirks ABI details, such as ADDR_LIMIT_32BIT,

We've had a similar discussion about JIT software on ARM64, which has a wide
range of supported page table layouts and some software wants to limit that
to a specific number.

I don't remember the outcome of that discussion, but I'm adding a few people
to Cc that might remember.

There have also been some discussions in the past to make the depth of the
page table a per-task decision on s390, since you may have some tasks that
run just fine with two or three levels of paging while another task actually
wants the full 64-bit address space. I wonder how much extra work this would
be on top of the boot-time option.