[RFC PATCH 00/10] arm64: allow virtually mapped stacks to be enabled

[Mark Rutland]

Hi Ard,

On Wed, Jul 12, 2017 at 03:44:13PM +0100, Ard Biesheuvel wrote:
> This is a fairly quick'n'dirty attempt at enabling virtually mapped
> stacks for arm64, after learning from Mark yesterday that progress
> had more or less stalled on that front.

Thanks for putting this together. If nothing else, this work needed a
good kick.

I had some rought comments on this, but in the process of wiring those
up, I ended up writing an alternative [1] by cobblnig together prior
attempts. Apologies for the NIH.

> Since having actual patches to poke at is infinitely better than having
> abstract discussions about how to approach it, I threw some code together
> that:
> 1. frees up sp_el0 by promoting register x18 to 'current' pointer while in
>    the kernel; flames welcome :-) (#7)
> 2. preparatory work to allow 1., i.e., to free up x18 and preserve/restore it
>    correctly as a platform register (#2, #3, #4, #5, #6)

>From past experience [2], I know that Will is not a fan of reserving a
GPR like this. There are a couple of other ways we can free up SP_EL0,
though, so that isn't the end of the world.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2017-July/518434.html
[2] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/this-cpu-reg

> 3. dump the entire task stack if regs->sp points elsewhere (#8)

This sounds useful, but it's liable to fill a scrollbuffer and/or take a
while to dump (especially with 64K stacks), so we might want a boot-time
option to turn that on/off. 

Thanks,
Mark.