Re: [PATCH] riscv: Gate FUNCTION_ALIGNMENT_4B on DYNAMIC_FTRACE

[Paul Walmsley <pjw@kernel.org>]

On Mon, 6 Jul 2026, Rui Qi wrote:

> The FUNCTION_ALIGNMENT_4B select forces the whole kernel to be built
> with -fmin-function-alignment=4. This alignment is only needed so the
> patchable-function-entry NOPs, which arch/riscv/Makefile emits under
> CONFIG_DYNAMIC_FTRACE, can be patched reliably on RISCV_ISA_C=y builds
> where compressed instructions otherwise allow 2-byte function
> alignment.
> 
> The select is currently gated on HAVE_DYNAMIC_FTRACE, a capability bit
> that is selected whenever the toolchain supports dynamic ftrace, rather
> than on whether tracing is actually enabled. As a result every
> RISCV_ISA_C=y build gets 4-byte function alignment across the entire
> kernel even when function tracing is disabled, needlessly growing the
> kernel image and wasting instruction cache for a feature that is not
> in use.
> 
> Gate the select on DYNAMIC_FTRACE instead, matching the condition under
> which arch/riscv/Makefile emits -fpatchable-function-entry, so the
> alignment is only applied when it is actually needed.
> 
> Fixes: c41bf4326c7b ("riscv: ftrace: align patchable functions to 4 Byte boundary")
> Signed-off-by: Rui Qi <qirui.001@bytedance.com>

Thanks, queued for v7.2-rc.

- Paul


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