On Sat, Sep 19, 2020 at 07:49:26PM -0400, Nicolas Pitre wrote:

On Fri, 18 Sep 2020, Ard Biesheuvel wrote:

quoted

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.

However, that code has to run _before_ the virtual mappings are setup,
which makes C code out of the question, unless we build a separate
executable binary and then insert it into the kernel image.  So, sorry,
it's not going to be practical to rewrite it in C.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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