svn commit: r328893 - head/lib/libc/mips/gen

[John Baldwin]

Author: jhb
Date: Mon Feb  5 18:10:28 2018
New Revision: 328893
URL: https://svnweb.freebsd.org/changeset/base/328893

Log:
  Fix makecontext() on MIPS O32.
  
  The GP register can be clobbered by the callback, so save it in S1
  while invoking the callback function.
  
  While here, add a comment expounding on the treatment of GP for the
  various ABIs and the assumptions made.
  
  Reviewed by:	jmallett (earlier version)
  Sponsored by:	DARPA / AFRL
  Differential Revision:	https://reviews.freebsd.org/D14179

Modified:
  head/lib/libc/mips/gen/_ctx_start.S

Modified: head/lib/libc/mips/gen/_ctx_start.S
==============================================================================
--- head/lib/libc/mips/gen/_ctx_start.S	Mon Feb  5 18:06:54 2018	(r328892)
+++ head/lib/libc/mips/gen/_ctx_start.S	Mon Feb  5 18:10:28 2018	(r328893)
@@ -28,11 +28,25 @@
 __FBSDID("$FreeBSD$");
 
 /*
- * XXX gp?
+ * This requires makecontext() to setup a valid GP for locating
+ * _ctx_done rather than deriving GP from T9 on entry.  Currently this
+ * uses the GP inherited from getcontext() assuming that getcontext()
+ * is in the same shared object as _ctx_done().  For N32 and N64, GP
+ * is caller-save so will be preserved across the call to the callback
+ * function.  For O32, GP is callee-save, so save it in a different
+ * caller-save register (S1) while invoking the callback.  This is
+ * done instead of the usual SETUP_GP/SAVE_GP to avoid disturbing the
+ * stack frame setup by makecontext() for the callback function.
  */
 ENTRY(_ctx_start)
+#ifdef __mips_o32
+	move	s1, gp
+#endif
 	jalr	t9
 
+#ifdef __mips_o32
+	move	gp, s1
+#endif
 	move	a0, s0
 	PTR_LA	t9, _ctx_done
 	jalr	t9