linux-kernel - [PATCH V2 2/3] riscv: ftrace: Remove wasted nops for !RISCV_ISA

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Date:   Tue, 20 Sep 2022 23:49:09 -0400
From:   guoren@...nel.org
To:     arnd@...db.de, palmer@...osinc.com, rostedt@...dmis.org,
        andy.chiu@...ive.com, greentime.hu@...ive.com, zong.li@...ive.com,
        jrtc27@...c27.com, mingo@...hat.com, palmer@...belt.com
Cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, guoren@...nel.org,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: [PATCH V2 2/3] riscv: ftrace: Remove wasted nops for !RISCV_ISA_C

From: Guo Ren <guoren@...ux.alibaba.com>

When CONFIG_RISCV_ISA_C=n, -fpatchable-function-entry=8 would generate
more nops than we expect. Because it treat nop opcode as 0x00000013
instead of 0x0001.

Dump of assembler code for function dw_pcie_free_msi:
   0xffffffff806fce94 <+0>:     sd      ra,-8(sp)
   0xffffffff806fce98 <+4>:     auipc   ra,0xff90f
   0xffffffff806fce9c <+8>:     jalr    -684(ra) # 0xffffffff8000bbec
<ftrace_caller>
   0xffffffff806fcea0 <+12>:    ld      ra,-8(sp)
   0xffffffff806fcea4 <+16>:    nop /* wasted */
   0xffffffff806fcea8 <+20>:    nop /* wasted */
   0xffffffff806fceac <+24>:    nop /* wasted */
   0xffffffff806fceb0 <+28>:    nop /* wasted */
   0xffffffff806fceb4 <+0>:     addi    sp,sp,-48
   0xffffffff806fceb8 <+4>:     sd      s0,32(sp)
   0xffffffff806fcebc <+8>:     sd      s1,24(sp)
   0xffffffff806fcec0 <+12>:    sd      s2,16(sp)
   0xffffffff806fcec4 <+16>:    sd      s3,8(sp)
   0xffffffff806fcec8 <+20>:    sd      ra,40(sp)
   0xffffffff806fcecc <+24>:    addi    s0,sp,48

Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
Signed-off-by: Guo Ren <guoren@...nel.org>
---
 arch/riscv/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 3fa8ef336822..f32844f545d6 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -11,7 +11,11 @@ LDFLAGS_vmlinux :=
 ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
 	LDFLAGS_vmlinux := --no-relax
 	KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
+ifeq ($(CONFIG_RISCV_ISA_C),y)
 	CC_FLAGS_FTRACE := -fpatchable-function-entry=8
+else
+	CC_FLAGS_FTRACE := -fpatchable-function-entry=4
+endif
 endif
 
 ifeq ($(CONFIG_CMODEL_MEDLOW),y)
-- 
2.36.1