linux-kernel - [PATCH v2 17/17] powerpc/32: Don't add dummy frames when calling trace_hardirqs

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Message-Id: <8f513e30d3fc875263befdaf38f4f80c3165c671.1553877076.git.christophe.leroy@c-s.fr>
Date:   Fri, 29 Mar 2019 16:38:39 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v2 17/17] powerpc/32: Don't add dummy frames when calling
 trace_hardirqs_on/off

No need to add dummy frames when calling trace_hardirqs_on or
trace_hardirqs_off. GCC properly handles empty stacks.

In addition, powerpc doesn't set CONFIG_FRAME_POINTER, therefore
__builtin_return_address(1..) returns NULL at all time. So the
dummy frames are definitely unneeded here.

In the meantime, avoid reading memory for loading r1 with a value
we already know.

Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
---
 arch/powerpc/kernel/entry_32.S | 16 ++--------------
 1 file changed, 2 insertions(+), 14 deletions(-)

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index c648b75f41a3..c1e9be5cba10 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S
@@ -237,12 +237,7 @@ transfer_to_handler_cont:
 
 reenable_mmu:
 	/*
-	 * The trace_hardirqs_off will use CALLER_ADDR0 and CALLER_ADDR1.
-	 * If from user mode there is only one stack frame on the stack, and
-	 * accessing CALLER_ADDR1 will cause oops. So we need create a dummy
-	 * stack frame to make trace_hardirqs_off happy.
-	 *
-	 * This is handy because we also need to save a bunch of GPRs,
+	 * We save a bunch of GPRs,
 	 * r3 can be different from GPR3(r1) at this point, r9 and r11
 	 * contains the old MSR and handler address respectively,
 	 * r4 & r5 can contain page fault arguments that need to be passed
@@ -933,18 +928,11 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_TYPE_47x)
 	 */
 	andi.	r10,r9,MSR_EE
 	beq	1f
-	/*
-	 * Since the ftrace irqsoff latency trace checks CALLER_ADDR1,
-	 * which is the stack frame here, we need to force a stack frame
-	 * in case we came from user space.
-	 */
 	stwu	r1,-32(r1)
 	mflr	r0
 	stw	r0,4(r1)
-	stwu	r1,-32(r1)
 	bl	trace_hardirqs_on
-	lwz	r1,0(r1)
-	lwz	r1,0(r1)
+	addi	r1, r1, 32
 	lwz	r9,_MSR(r1)
 1:
 #endif /* CONFIG_TRACE_IRQFLAGS */
-- 
2.13.3