On Friday, December 9, 2016 6:01:30 AM CET Ingo Molnar wrote:

quoted

  - 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, 
ADDR_LIMIT_3GB, ADDR_COMPAT_LAYOUT, READ_IMPLIES_EXEC, etc.

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.

	Arnd