[PATCH] powerpc: workaround clang codegen bug in dcbz

From: Nick Desaulniers <ndesaulniers@google.com>
To: mpe@ellerman.id.au
Cc: christophe.leroy@c-s.fr, segher@kernel.crashing.org,
	arnd@arndb.de, Nick Desaulniers <ndesaulniers@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	kbuild test robot <lkp@intel.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com
Subject: [PATCH] powerpc: workaround clang codegen bug in dcbz
Date: Mon, 29 Jul 2019 13:25:41 -0700	[thread overview]
Message-ID: <20190729202542.205309-1-ndesaulniers@google.com> (raw)

Commit 6c5875843b87 ("powerpc: slightly improve cache helpers") exposed
what looks like a codegen bug in Clang's handling of `%y` output
template with `Z` constraint. This is resulting in panics during boot
for 32b powerpc builds w/ Clang, as reported by our CI.

Add back the original code that worked behind a preprocessor check for
__clang__ until we can fix LLVM.

Further, it seems that clang allnoconfig builds are unhappy with `Z`, as
reported by 0day bot. This is likely because Clang warns about inline
asm constraints when the constraint requires inlining to be semantically
valid.

Link: https://bugs.llvm.org/show_bug.cgi?id=42762
Link: https://github.com/ClangBuiltLinux/linux/issues/593
Link: https://lore.kernel.org/lkml/20190721075846.GA97701@archlinux-threadripper/
Debugged-by: Nathan Chancellor <natechancellor@gmail.com>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Reported-by: kbuild test robot <lkp@intel.com>
Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Alternatively, we could just revert 6c5875843b87. It seems that GCC
generates the same code for these functions for out of line versions.
But I'm not sure how the inlined code generated would be affected.

 arch/powerpc/include/asm/cache.h | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/arch/powerpc/include/asm/cache.h b/arch/powerpc/include/asm/cache.h
index b3388d95f451..72983da94dce 100644
--- a/arch/powerpc/include/asm/cache.h
+++ b/arch/powerpc/include/asm/cache.h
@@ -105,6 +105,30 @@ extern void _set_L3CR(unsigned long);
 #define _set_L3CR(val)	do { } while(0)
 #endif
 
+/*
+ * Workaround for https://bugs.llvm.org/show_bug.cgi?id=42762.
+ */
+#ifdef __clang__
+static inline void dcbz(void *addr)
+{
+	__asm__ __volatile__ ("dcbz 0, %0" : : "r"(addr) : "memory");
+}
+
+static inline void dcbi(void *addr)
+{
+	__asm__ __volatile__ ("dcbi 0, %0" : : "r"(addr) : "memory");
+}
+
+static inline void dcbf(void *addr)
+{
+	__asm__ __volatile__ ("dcbf 0, %0" : : "r"(addr) : "memory");
+}
+
+static inline void dcbst(void *addr)
+{
+	__asm__ __volatile__ ("dcbst 0, %0" : : "r"(addr) : "memory");
+}
+#else
 static inline void dcbz(void *addr)
 {
 	__asm__ __volatile__ ("dcbz %y0" : : "Z"(*(u8 *)addr) : "memory");
@@ -124,6 +148,7 @@ static inline void dcbst(void *addr)
 {
 	__asm__ __volatile__ ("dcbst %y0" : : "Z"(*(u8 *)addr) : "memory");
 }
+#endif /* __clang__ */
 #endif /* !__ASSEMBLY__ */
 #endif /* __KERNEL__ */
 #endif /* _ASM_POWERPC_CACHE_H */
-- 
2.22.0.709.g102302147b-goog

