On Fri, Dec 09, 2016 at 11:24:12AM +0100, Arnd Bergmann wrote:

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

quoted

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.

The arm64 kernel supports several user VA space configurations (though
commonly 39 and 48-bit) and has had these from the initial port. We
realised that certain JITs (e.g.
https://bugzilla.mozilla.org/show_bug.cgi?id=1143022) and IIRC LLVM
assume a 47-bit user VA but AFAICT, most have been fixed.

ARMv8.1 also supports 52-bit VA (though only with 64K pages and we
haven't added support for it yet). However, it's likely that if we make
a 52-bit TASK_SIZE this the default, we will break some user
assumptions. While arguably that's not necessarily ABI, if user relies
on a 47 or 48-bit VA the kernel shouldn't break it. So I'm strongly
inclined to make the 52-bit TASK_SIZE an opt-in on arm64.

-- 
Catalin

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