linux-kernel - [PATCH 4.9 21/66] powerpc: Fix build failure with clang due to BUILD_BUG

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Message-Id: <20170207124529.207534517@linuxfoundation.org>
Date:   Tue,  7 Feb 2017 13:58:55 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Anton Blanchard <anton@...ba.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: [PATCH 4.9 21/66] powerpc: Fix build failure with clang due to BUILD_BUG_ON()

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Michael Ellerman <mpe@...erman.id.au>

commit b5fa0f7f88edcde37df1807fdf9ff10ec787a60e upstream.

Anton says: In commit 4db7327194db ("powerpc: Add option to use jump
label for cpu_has_feature()") and commit c12e6f24d413 ("powerpc: Add
option to use jump label for mmu_has_feature()") we added:

  BUILD_BUG_ON(!__builtin_constant_p(feature))

to cpu_has_feature() and mmu_has_feature() in order to catch usage
issues (such as cpu_has_feature(cpu_has_feature(X), which has happened
once in the past). Unfortunately LLVM isn't smart enough to resolve
this, and it errors out.

I work around it in my clang/LLVM builds of the kernel, but I have just
discovered that it causes a lot of issues for the bcc (eBPF) trace tool
(which uses LLVM).

For now just #ifdef it away for clang builds.

Fixes: 4db7327194db ("powerpc: Add option to use jump label for cpu_has_feature()")
Fixes: c12e6f24d413 ("powerpc: Add option to use jump label for mmu_has_feature()")
Reported-by: Anton Blanchard <anton@...ba.org>
Tested-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/powerpc/include/asm/cpu_has_feature.h |    2 ++
 arch/powerpc/include/asm/mmu.h             |    2 ++
 2 files changed, 4 insertions(+)

--- a/arch/powerpc/include/asm/cpu_has_feature.h
+++ b/arch/powerpc/include/asm/cpu_has_feature.h
@@ -23,7 +23,9 @@ static __always_inline bool cpu_has_feat
 {
 	int i;

+#ifndef __clang__ /* clang can't cope with this */
 	BUILD_BUG_ON(!__builtin_constant_p(feature));
+#endif

 #ifdef CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG
 	if (!static_key_initialized) {
--- a/arch/powerpc/include/asm/mmu.h
+++ b/arch/powerpc/include/asm/mmu.h
@@ -160,7 +160,9 @@ static __always_inline bool mmu_has_feat
 {
 	int i;

+#ifndef __clang__ /* clang can't cope with this */
 	BUILD_BUG_ON(!__builtin_constant_p(feature));
+#endif

 #ifdef CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG
 	if (!static_key_initialized) {