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

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

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

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.

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