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From:   Arnd Bergmann <arnd@arndb.de>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org
Cc:     Arnd Bergmann <arnd@arndb.de>, "H. Peter Anvin" <hpa@zytor.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Zhenzhong Duan <zhenzhong.duan@oracle.com>,
        Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Juergen Gross <jgross@suse.com>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: [PATCH] x86: work around clang IAS bug referencing __force_order
Date:   Wed, 27 May 2020 15:53:19 +0200
Message-Id: <20200527135329.1172644-1-arnd@arndb.de>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

When using the clang integrated assembler, we get a reference
to __force_order that should normally get ignored in a few
rare cases:

ERROR: modpost: "__force_order" [drivers/cpufreq/powernow-k6.ko] undefined!

Add a 'static' definition so any file in which this happens can
have a local copy.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 arch/x86/boot/compressed/pgtable_64.c | 2 ++
 arch/x86/include/asm/special_insns.h  | 7 +++++++
 2 files changed, 9 insertions(+)

diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index c8862696a47b..8595194cea41 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -12,7 +12,9 @@
  * It is not referenced from the code, but GCC < 5 with -fPIE would fail
  * due to an undefined symbol. Define it to make these ancient GCCs work.
  */
+#ifndef CONFIG_CC_IS_CLANG
 unsigned long __force_order;
+#endif
 
 #define BIOS_START_MIN		0x20000U	/* 128K, less than this is insane */
 #define BIOS_START_MAX		0x9f000U	/* 640K, absolute maximum */
diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 82436cb04ccf..7081e587c1ea 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -16,8 +16,15 @@
  * A memory clobber would solve the problem, but would prevent reordering of
  * all loads stores around it, which can hurt performance. Solution is to
  * use a variable and mimic reads and writes to it to enforce serialization
+ *
+ * Clang sometimes fails to kill the reference to the dummy variable, so
+ * provide an actual copy.
  */
+#ifdef CONFIG_CC_IS_CLANG
+static unsigned long __force_order;
+#else
 extern unsigned long __force_order;
+#endif
 
 void native_write_cr0(unsigned long val);
 
-- 
2.26.2