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

[Catalin Marinas]

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:
> > >   - 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