Bug#834505: arm64 boot failure with large physical memory range

[Leif Lindholm <leif.lindholm@linaro.org>]

On Fri, Aug 19, 2016 at 02:04:07AM +0100, Ben Hutchings wrote:
> > > > So please enable CONFIG_ARM64_VA_BITS_48 for arm64 kernels.
> > > > 
> > > > Note: this change _will_ cause breakage in certain userland software
> > > > making non-portable assumptions about available of top address bits
> > > > for pointer tagging - including luajit and mozilla-js (and hence
> > > > everything using mozilla-js).
> [...]
> 
> Could we possibly work around that by reducing
> CONFIG_ARCH_MMAP_RND_BITS_MAX?  (That's not directly configurable; it
> requires patching arch/arm64/Kconfig.)

I think this would be opening up a real can of worms. Not all sizes
are supported by the architecture, and only certain VA_BITS/pagesize
combinations work in the kernel.

We could switch to 42-bit VA, but that would require switching to 64K
pagesize, which would be an even huger can.

Proposed workarounds have included copying (in the stub) the kernel
and initrd to live closer together, but this still leaves potential
issues for (for example) ACPI tables.

/
    Leif