[PATCH v2 00/10] ARM: p2v: reduce min alignment to 2 MiB

[Nicolas Pitre]

On Mon, 21 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.
> 
> Patch #1 fixes a BE8 bug in the existing code. 
> 
> Patch #2 incorporates a patch from a different series [1] that introduces
> the adr_l and str_l macros, for use in subsequent patches.
> 
> Patch #3 moves the p2v patching code into a separate assembler source file,
> as suggested by Nico.
> 
> Patches #4 to #8 perform some preparatory clean up to make the real changes
> easier to review, and more maintainable going forward.
> 
> Patch #9 switches the ARM/LPAE and Thumb2 p2v patchable sequences to MOVW,
> so that we can extend the size of the immediate field in a later patch.
> 
> Patch #10 increases the size of the low order immediate to 11 bits, so that
> we can deal with any physical to virtual displacement that is a multiple of
> 2 MiB.
> 
> Tested in QEMU using various permutations of the the following configuration
> options:
> - ARM mode vs Thumb2 mode
> - LE vs BE8
> - LPAE vs non-LPAE
> - 3/1 split, 2/2 split, 3/1 opt split
> 
> with the following diff applied to decompress the kernel at an address which
> is not 16 MiB aligned:
> 
> --- a/arch/arm/boot/compressed/head.S
> +++ b/arch/arm/boot/compressed/head.S
> @@ -275,6 +275,7 @@ not_angel:
>                 and     r4, r4, #0xf8000000
>                 /* Determine final kernel image address. */
>                 add     r4, r4, #TEXT_OFFSET
> +               add     r4, r4, #SZ_2M
>  #else
>                 ldr     r4, =zreladdr
>  #endif
> 
> Changes since RFC/RFT:
> - new patches #1 - #3 and #8
> - drop bogus patching of ADD/SUB instructions, which is unnecessary, and only
>   happens to work when the p2v displacement is exactly 2 GiB like on QEMU when
>   running a 3/1 split
> - use a copious amount of comments to make the code more understandable and
>   maintainable
> 
> Cc: Zhen Lei <thunder.leizhen at huawei.com>
> Cc: Russell King <rmk+kernel at armlinux.org.uk>
> Cc: Linus Walleij <linus.walleij at linaro.org>
> Cc: Nicolas Pitre <nico at fluxnic.net>

Acked-by: Nicolas Pitre <nico at fluxnic.net>


> 
> [0] https://lore.kernel.org/linux-arm-kernel/20200915015204.2971-1-thunder.leizhen@huawei.com/
> [1] https://lore.kernel.org/linux-arm-kernel/20200914095706.3985-1-ardb@kernel.org/
> 
> Ard Biesheuvel (10):
>   ARM: p2v: fix handling of LPAE translation in BE mode
>   ARM: assembler: introduce adr_l, ldr_l and str_l macros
>   ARM: p2v: move patching code to separate assembler source file
>   ARM: p2v: factor out shared loop processing
>   ARM: p2v: factor out BE8 handling
>   ARM: p2v: drop redundant 'type' argument from __pv_stub
>   ARM: p2v: use relative references in patch site arrays
>   ARM: p2v: simplify __fixup_pv_table()
>   ARM: p2v: switch to MOVW for Thumb2 and ARM/LPAE
>   ARM: p2v: reduce p2v alignment requirement to 2 MiB
> 
>  arch/arm/Kconfig                 |   2 +-
>  arch/arm/include/asm/assembler.h |  84 +++++++
>  arch/arm/include/asm/memory.h    |  57 +++--
>  arch/arm/kernel/Makefile         |   1 +
>  arch/arm/kernel/head.S           | 142 ------------
>  arch/arm/kernel/phys2virt.S      | 238 ++++++++++++++++++++
>  6 files changed, 364 insertions(+), 160 deletions(-)
>  create mode 100644 arch/arm/kernel/phys2virt.S
> 
> -- 
> 2.17.1
> 
>