Re: JITs and 52-bit VA

[Dave Hansen <dave.hansen@intel.com>]

On 06/22/2016 12:20 PM, Andy Lutomirski wrote:
>>> >> As an example, a 32-bit x86 program really could have something mapped
>>> >> above the 32-bit boundary.  It just wouldn't be useful, but the kernel
>>> >> should still understand that it's *user* memory.
>>> >>
>>> >> So you'd have PR_SET_MMAP_LIMIT and PR_GET_MMAP_LIMIT or similar instead.
>> >
>> > +1. Also it might be (not sure though, just guessing) suitable to do such
>> > thing via memory cgroup controller, instead of carrying this limit per
>> > each process (or task structure/vma or mm).
> I think we'll want this per mm.  After all, a high-VA-limit-aware bash
> should be able run high-VA-unaware programs without fiddling with
> cgroups.

Yeah, cgroups don't make a lot of sense.

On x86, the 48-bit virtual address is even hard-coded in the ABI[1].  So
we can't change *any* program's layout without either breaking the ABI
or having it opt in.

But, we're also lucky to only have one VA layout since day one.

1. www.x86-64.org/documentation/abi.pdf - a??... Therefore, conforming
processes may only use addresses from 0x00000000 00000000 to 0x00007fff
ffffffff .a??


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