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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47f464c30a9sm2251049f8f.31.2026.07.13.14.23.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Jul 2026 14:23:21 -0700 (PDT) Date: Mon, 13 Jul 2026 22:23:20 +0100 From: David Laight To: "Rui Qi" Cc: , , , "Alexandre Ghiti" , "Andy Chiu" , =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= , "open list:RISC-V ARCHITECTURE" , "open list" Subject: Re: [PATCH] riscv: Gate FUNCTION_ALIGNMENT_4B on DYNAMIC_FTRACE Message-ID: <20260713222320.12ad21f3@pumpkin> In-Reply-To: <20260706130415.463682-1-qirui.001@bytedance.com> References: <20260706130415.463682-1-qirui.001@bytedance.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 6 Jul 2026 21:04:14 +0800 "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. I'd have thought that 4-byte aligning functions would increase performance. (cache-line aligning is likely to be best, but that adds a lot more bloat.) David > > 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 > --- > arch/riscv/Kconfig | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig > index 66638fc9ba43..0c00f6aa715a 100644 > --- a/arch/riscv/Kconfig > +++ b/arch/riscv/Kconfig > @@ -158,7 +158,7 @@ config RISCV > select HAVE_DEBUG_KMEMLEAK > select HAVE_DMA_CONTIGUOUS if MMU > select HAVE_DYNAMIC_FTRACE if MMU && (CLANG_SUPPORTS_DYNAMIC_FTRACE || GCC_SUPPORTS_DYNAMIC_FTRACE) > - select FUNCTION_ALIGNMENT_4B if HAVE_DYNAMIC_FTRACE && RISCV_ISA_C > + select FUNCTION_ALIGNMENT_4B if DYNAMIC_FTRACE && RISCV_ISA_C > select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS if HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS > select HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS if (DYNAMIC_FTRACE_WITH_ARGS && !CFI) > select HAVE_DYNAMIC_FTRACE_WITH_ARGS if HAVE_DYNAMIC_FTRACE