Re: [PATCH 0/6] arm64: alternatives: switch most used alternatives to callbacks

[Catalin Marinas <catalin.marinas@arm.com>]

Hi Ada,

On Mon, Sep 28, 2026 at 02:30:28PM +0100, Ada Couprie Diaz wrote:
> All numbers are on v7.3-rc4 building defconfig with GCC 13.3.0.
> Only the patches mentioned are applied on each line.
> 
> |   Patches    |  Size (B) |	|    Patch    | Size (B) |
> | Base vmlinux | 172826400 |	|  Base Image | 52374016 |
> |          1-3 |    -71672 |	|         1-3 |   -65536 |
> |            4 |    -71704 |    |           4 |   -65536 |
> |          1-4 |    -64712 |	|         1-4 |   -65536 |
> |          5-6 |    +68008 |	|         5-6 |       -0 |
> |  All patches |    + 3240 |    | All patches |   -65536 |
> 
> The impact on alternatives is two-fold :
> 1. As expected, all alternatives for the two I/O workarounds and
>    97% of the `ARM64_HAS_VIRT_HOST_EXTN` alternatives are converted to
>    callbacks, saving 85848 bytes (20 pages).
> 2. The raw number of alternatives *increases* by about 3% (250 new entries)

TBH, I don't think the saving is worth the additional complexity.

I wonder, could we instead move the replacement instructions to a
separate section we can drop after patching? I can see x86 uses a
separate .altinstr_replacement. Not sure what restrictions we have on
arm64, e.g. if it's placed too far.

That said, are some of the other patches worth picking up as cleanups?

-- 
Catalin