svn commit: r336349 - head/sys/sys

[Andrew Turner]

Author: andrew
Date: Mon Jul 16 18:21:29 2018
New Revision: 336349
URL: https://svnweb.freebsd.org/changeset/base/336349

Log:
  Don't use the static keyword with DPCPU defines in arm64 modules.
  
  On arm64 compiler will create PC-relative loads and stores for static data.
  This means it doesn't emit a relocation. Unfortunately the in-kernel linker
  expects there to be one for DPCPU defines so it can modify its value so the
  code will use the correct DPCPU region.
  
  To workaround the lack of a relocation with static data remove it when
  building modules on arm64. The kernel is unaffected as it doesn't rely on
  modifying these relocations to find the data.
  
  PR:		225684
  Reported by:	Johannes Lundberg <johalun0 at gmail.com>
  Reported by:	Jose Luis Duran <jlduran at gmail.com>
  Reported by:	Greg V <greg at unrelenting.technology>
  Reviewed by:	bz
  Sponsored by:	ABT Systems Ltd
  Differential Revision:	https://reviews.freebsd.org/D16145

Modified:
  head/sys/sys/pcpu.h

Modified: head/sys/sys/pcpu.h
==============================================================================
--- head/sys/sys/pcpu.h	Mon Jul 16 15:39:33 2018	(r336348)
+++ head/sys/sys/pcpu.h	Mon Jul 16 18:21:29 2018	(r336349)
@@ -84,8 +84,26 @@ extern uintptr_t dpcpu_off[];
 /* struct _hack is to stop this from being used with the static keyword. */
 #define	DPCPU_DEFINE(t, n)	\
     struct _hack; t DPCPU_NAME(n) __section(DPCPU_SETNAME) __used
+#if defined(KLD_MODULE) && defined(__aarch64__)
+/*
+ * On some architectures the compiler will use PC-relative load to
+ * find the address of DPCPU data with the static keyword. We then
+ * use this to find the offset of the data in a per-CPU region.
+ * This works for in the kernel as we can allocate the space ahead
+ * of time, however modules need to allocate a sepatate space and
+ * then use relocations to fix the address of the data. As
+ * PC-relative data doesn't have a relocation there is nothing for
+ * the kernel module linker to fix so data is accessed from the
+ * wrong location.
+ *
+ * This is a workaround until a better solution can be found.
+*/
 #define	DPCPU_DEFINE_STATIC(t, n)	\
+    t DPCPU_NAME(n) __section(DPCPU_SETNAME) __used
+#else
+#define	DPCPU_DEFINE_STATIC(t, n)	\
     static t DPCPU_NAME(n) __section(DPCPU_SETNAME) __used
+#endif
 
 /*
  * Accessors with a given base.