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Message-Id: <1650262058.altfknhzto.naveen@linux.ibm.com>
Date: Mon, 18 Apr 2022 11:51:16 +0530
From: "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To: Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Christophe Leroy <christophe.leroy@...roup.eu>,
Ingo Molnar <mingo@...hat.com>,
Michael Ellerman <mpe@...erman.id.au>,
Paul Mackerras <paulus@...ba.org>,
Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v1 07/22] powerpc/ftrace: Use patch_instruction() return
directly
Christophe Leroy wrote:
> Instead of returning -EPERM when patch_instruction() fails,
> just return what patch_instruction returns.
>
> That simplifies ftrace_modify_code():
>
> 0: 94 21 ff c0 stwu r1,-64(r1)
> 4: 93 e1 00 3c stw r31,60(r1)
> 8: 7c 7f 1b 79 mr. r31,r3
> c: 40 80 00 30 bge 3c <ftrace_modify_code+0x3c>
> 10: 93 c1 00 38 stw r30,56(r1)
> 14: 7c 9e 23 78 mr r30,r4
> 18: 7c a4 2b 78 mr r4,r5
> 1c: 80 bf 00 00 lwz r5,0(r31)
> 20: 7c 1e 28 40 cmplw r30,r5
> 24: 40 82 00 34 bne 58 <ftrace_modify_code+0x58>
> 28: 83 c1 00 38 lwz r30,56(r1)
> 2c: 7f e3 fb 78 mr r3,r31
> 30: 83 e1 00 3c lwz r31,60(r1)
> 34: 38 21 00 40 addi r1,r1,64
> 38: 48 00 00 00 b 38 <ftrace_modify_code+0x38>
> 38: R_PPC_REL24 patch_instruction
>
> Before:
>
> 0: 94 21 ff c0 stwu r1,-64(r1)
> 4: 93 e1 00 3c stw r31,60(r1)
> 8: 7c 7f 1b 79 mr. r31,r3
> c: 40 80 00 4c bge 58 <ftrace_modify_code+0x58>
> 10: 93 c1 00 38 stw r30,56(r1)
> 14: 7c 9e 23 78 mr r30,r4
> 18: 7c a4 2b 78 mr r4,r5
> 1c: 80 bf 00 00 lwz r5,0(r31)
> 20: 7c 08 02 a6 mflr r0
> 24: 90 01 00 44 stw r0,68(r1)
> 28: 7c 1e 28 40 cmplw r30,r5
> 2c: 40 82 00 48 bne 74 <ftrace_modify_code+0x74>
> 30: 7f e3 fb 78 mr r3,r31
> 34: 48 00 00 01 bl 34 <ftrace_modify_code+0x34>
> 34: R_PPC_REL24 patch_instruction
> 38: 80 01 00 44 lwz r0,68(r1)
> 3c: 20 63 00 00 subfic r3,r3,0
> 40: 83 c1 00 38 lwz r30,56(r1)
> 44: 7c 63 19 10 subfe r3,r3,r3
> 48: 7c 08 03 a6 mtlr r0
> 4c: 83 e1 00 3c lwz r31,60(r1)
> 50: 38 21 00 40 addi r1,r1,64
> 54: 4e 80 00 20 blr
>
> It improves ftrace activation/deactivation duration by about 3%.
>
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
> arch/powerpc/kernel/trace/ftrace.c | 5 +----
> 1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/kernel/trace/ftrace.c b/arch/powerpc/kernel/trace/ftrace.c
> index 98e82fa4980f..1b05d33f96c6 100644
> --- a/arch/powerpc/kernel/trace/ftrace.c
> +++ b/arch/powerpc/kernel/trace/ftrace.c
> @@ -78,10 +78,7 @@ ftrace_modify_code(unsigned long ip, ppc_inst_t old, ppc_inst_t new)
> }
>
> /* replace the text with the new text */
> - if (patch_instruction((u32 *)ip, new))
> - return -EPERM;
> -
> - return 0;
> + return patch_instruction((u32 *)ip, new);
I think the reason we were returning -EPERM is so that ftrace_bug() can
throw the right error message. That will change due to this patch,
though I'm not sure how much it matters. -EFAULT and -EPERM seem to
print almost the same error message.
- Naveen
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