[RFC] arm64: defconfig: enable 48-bit VA by default

[Ard Biesheuvel]

On 30 July 2015 at 18:32, Stuart Yoder <stuart.yoder at freescale.com> wrote:
>
>
>> -----Original Message-----
>> From: Catalin Marinas [mailto:catalin.marinas at arm.com]
>>
>> For the time being, I would keep defconfig to 39-bit VA. If we start
>> seeing systems with lots of RAM (over 256GB), we'll probably change the
>> defconfig.
>
> Somone who follows the "Principles" doc "correctly" will hit an issue
> with the linear memory map after 32GB of RAM, right?   2 GB in 1st alias,
> 30 GB in 2nd alias, and rest at the 544 GB offset.
>
> But, I guess it still remains to be seen who else will hit that
> and when.
>

Hi Stuart,

I think you may be overestimating the significance of defconfig. As
you pointed out, distros have their own configs (and many out of tree
patches) for the binary kernels they ship. Fedora currently ships with
64k pages because it was the only way to support Seattle before the
4-level 4kb pages implementation was introduced. They will likely keep
doing that, or switch to 4-level 4 kB pages since, as you pointed out,
it is fairly likely that servers have more than 32 GB DRAM, which
means that they need more than 256 GB of virtual address space in the
linear region if they follow the ARM recommendation.

Whether defconfig supports your platform optimally has nothing to do
with that. Of course, we should deal with the unexpected memory layout
gracefully, which is why Mark Rutland and myself proposed patches to
fix the panic you reported. But in a development context, I think it
is perfectly acceptable to simply load the kernel at 0x80_8000_0000,
and be able to run defconfig fine while losing just 2 GB of your 16 GB
at the low end. Of course, you would never ship a system like that,
but that is not what defconfig is meant to cater for.

Regards,
Ard.