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

[Ada Couprie Diaz <ada.coupriediaz@arm.com>]

Hi Catalin,

On 06/10/2026 17:28, Catalin Marinas wrote:
> 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.

Fair, I did expect it a bit but wanted to share if it could be useful.

> 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.

I think given the amount of alternatives we have now, there was some
distance issue, but I don't have the exact details. I can look into it !

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

Mh, I think patches 1 and 5 are nice small cleanups by themselves,
patch 2 is less obviously useful as-is but is a start on making the
insn framework safer to use for patching (which is not really reasonable
to tackle in one go anyway).

Thanks Catalin,
Kind regards
Ada