Re: [PATCH v7] arm64: implement support for static call trampolines

[Will Deacon]

Hi Carlos,

On Fri, Mar 13, 2026 at 06:18:52AM +0000, Carlos Llamas wrote:
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> 
> Implement arm64 support for the 'unoptimized' static call variety, which
> routes all calls through a single trampoline that is patched to perform a
> tail call to the selected function.
> 
> Since static call targets may be located in modules loaded out of direct
> branching range, we need to use a ADRP/ADD pair to load the branch target
> into R16 and use a branch-to-register (BR) instruction to perform an
> indirect call. Unlike on x86, there is no pressing need on arm64 to avoid
> indirect calls at all cost, but hiding it from the compiler as is done
> here does have some benefits:
> - the literal is located in .rodata, which gives us the same robustness
>   advantage that code patching does;
> - no performance hit on CFI enabled Clang builds that decorate compiler
>   emitted indirect calls with branch target validity checks.
> 
> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Signed-off-by: Carlos Llamas <cmllamas@xxxxxxxxxx>
> ---
> v7:
>   - Took Ard's v3 patch (as it leaves the code patching logic out) and
>     rebased it on top  of mainline 7.0-rc3.
>   - Dropped the changes to arch/arm64/lib/insn.c and instead switched to
>     the (now) existing aarch64_insn_write_literal_u64().
>   - Added the RET0 trampoline define which points to the generic stub
>     __static_call_return0.
>   - Made the HAVE_STATIC_CALL conditional on CFI as suggested by Ard.
>   - Added .type and .size sections to the trampoline definition to
>     support ABI tools.

Are you planning to respin this based on Ard's comments?

Cheers,

Will