[RFC/RFT PATCH 0/6] ARM: p2v: reduce min alignment to 2 MiB

[Ard Biesheuvel]

On Sun, 20 Sep 2020 at 01:49, Nicolas Pitre <nico at fluxnic.net> wrote:
>
> On Fri, 18 Sep 2020, Ard Biesheuvel wrote:
>
> > This series is inspired by Zhei Len's series [0], which updates the
> > ARM p2v patching code to optionally support p2v relative alignments
> > of as little as 64 KiB.
> >
> > Reducing this alignment is necessary for some specific Huawei boards,
> > but given that reducing this minimum alignment will make the boot
> > sequence more robust for all platforms, especially EFI boot, which
> > no longer relies on the 128 MB masking of the decompressor load address,
> > but uses firmware memory allocation routines to find a suitable spot
> > for the decompressed kernel.
> >
> > This series is not based on Zhei Len's code, but addresses the same
> > problem, and takes some feedback given in the review into account:
> > - use of a MOVW instruction to avoid two adds/adcs sequences when dealing
> >   with the carry on LPAE
> > - add support for Thumb2 kernels as well
> > - make the change unconditional - it will bit rot otherwise, and has value
> >   for other platforms as well.
> >
> > The first four patches are general cleanup and preparatory changes.
> > Patch #5 implements the switch to a MOVW instruction without changing
> > the minimum alignment.
> > Patch #6 reduces the minimum alignment to 2 MiB.
> >
> > Tested on QEMU in ARM/!LPAE, ARM/LPAE, Thumb2/!LPAE and Thumb2/LPAE modes.
>
> At this point I think this really ought to be split into a file of its
> own... and maybe even rewritten in C. Even though I wrote the original
> code, I no longer understand it without re-investing time into it. But
> in either cases the whole of head.S would need to have its registers
> shuffled first to move long lived values away from r0-r3,ip,lr to allow
> for standard function calls.
>

I agree with that in principle, however, running C code with a stack
with the MMU off is slightly risky.

I have managed to simplify the code a bit more (given that some
patching was not needed to begin with), and I can add some more
comments to head.S to annotate the actions.