Better define the flags in the mcontext_t and properly set the flags

when we create contexts. The meaning of the flags are documented in
<machine/ucontext.h>. I only list them here to help browsing the
commit logs:
	_MC_FLAGS_ASYNC_CONTEXT
	_MC_FLAGS_HIGHFP_VALID
	_MC_FLAGS_KSE_SET_MBOX
	_MC_FLAGS_RETURN_VALID
	_MC_FLAGS_SCRATCH_VALID

Yes, _MC_FLAGS_KSE_SET_MBOX is a hack and I'm proud of it :-)

1 files changed, 52 insertions, 2 deletions

diff --git a/sys/ia64/include/ucontext.h b/sys/ia64/include/ucontext.h
index e5fae65..66f8fff 100644
--- a/sys/ia64/include/ucontext.h
+++ b/sys/ia64/include/ucontext.h
@@ -33,10 +33,60 @@
 
 #include <machine/_regset.h>
 
+/*
+ * The mc_flags field provides the necessary clues when dealing with the gory
+ * details of ia64 specific contexts. A comprehensive explanation is added for
+ * everybody's sanity, including the author's.
+ *
+ * The first and foremost variation in the context is synchronous contexts
+ * (= synctx) versus asynchronous contexts (= asynctx). A synctx is created
+ * synchronously WRT program execution and has the advantage that none of the
+ * scratch registers have to be saved. They are assumed to be clobbered by the
+ * call to the function that creates the context. An asynctx needs to have the
+ * scratch registers preserved because it can describe any point in a thread's
+ * (or process') execution.
+ *
+ * Below a description of the flags and their meaning:
+ *
+ *	_MC_FLAGS_ASYNC_CONTEXT
+ *		If set, indicates that mc_scratch and mc_scratch_fp are both
+ *		valid. IFF not set, _MC_FLAGS_RETURN_VALID indicates if the
+ *		return registers are valid or not.
+ *	_MC_FLAGS_HIGHFP_VALID
+ *		If set, indicates that the high FP registers (f32-f127) are
+ *		valid. This flag is very likely not going to be set for any
+ *		sensible synctx, but is not explicitly disallowed. Any synctx
+ *		that has this flag may or may not have the high FP registers
+ *		restored. In short: don't do it.
+ *	_MC_FLAGS_KSE_SET_MBOX
+ *		This flag is special to setcontext(2) and swapcontext(2). It
+ *		instructs the kernel to write the value in mc_special.isr to
+ *		the memory address pointed to by mc_special.ifa. This allows
+ *		the kernel to switch to a new context in a KSE based threaded
+ *		program. Note that this is a non-srandard extension to the
+ *		otherwise standard system calls and use of this flag should be
+ *		limited to KSE.
+ *	_MC_FLAGS_RETURN_VALID
+ *		If set and _MC_FLAGS_ASYNC_CONTEXT is not set indicates that
+ *		the ABI defined return registers are valid. Both getcontext(2)
+ *		and swapcontext(2) need to save the system call return values.
+ *		Any synctx that does not have this flag may still have the
+ *		return registers restored from the context.
+ *	_MC_FLAGS_SCRATCH_VALID
+ *		If set and _MC_FLAGS_ASYNC_CONTEXT is not set indicates that
+ *		the scratch registers, but not the FP registers are valid.
+ *		This flag is set in contexts passed to signal handlers. This
+ *		flag is a superset of _MC_FLAGS_RETURN_VALID. If both flags
+ *		are set, this flag takes precedence.
+ */
+
 typedef struct __mcontext {
 	unsigned long		mc_flags;
-#define	IA64_MC_FLAGS_SCRATCH_VALID	1
-#define	IA64_MC_FLAGS_HIGHFP_VALID	2
+#define	_MC_FLAGS_ASYNC_CONTEXT		0x0001
+#define	_MC_FLAGS_HIGHFP_VALID		0x0002
+#define	_MC_FLAGS_KSE_SET_MBOX		0x0004
+#define	_MC_FLAGS_RETURN_VALID		0x0008
+#define	_MC_FLAGS_SCRATCH_VALID		0x0010
 	unsigned long		_reserved_;
 	struct _special		mc_special;
 	struct _callee_saved	mc_preserved;