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

[Nicolas Pitre]

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.


Nicolas