Add commentary about the new "asmlinkage_protect()" macro

It's really a pretty ugly thing to need, and some day it will hopefully
be obviated by teaching gcc about the magic calling conventions for the
low-level system call code, but in the meantime we can at least add big
honking comments about why we need these insane and strange macros.

I took my comments from my version of the macro, but I ended up deciding
to just pick Roland's version of the actual code instead (with his
prettier syntax that uses vararg macros).  Thus the previous two commits
that actually implement it.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/include/linux/linkage.h b/include/linux/linkage.h
index fe2a39c489b6..b163c5c40dbc 100644
--- a/include/linux/linkage.h
+++ b/include/linux/linkage.h
@@ -17,6 +17,19 @@
 # define asmregparm
 #endif
 
+/*
+ * This is used by architectures to keep arguments on the stack
+ * untouched by the compiler by keeping them live until the end.
+ * The argument stack may be owned by the assembly-language
+ * caller, not the callee, and gcc doesn't always understand
+ * that.
+ *
+ * We have the return value, and a maximum of six arguments.
+ *
+ * This should always be followed by a "return ret" for the
+ * protection to work (ie no more work that the compiler might
+ * end up needing stack temporaries for).
+ */
 #ifndef asmlinkage_protect
 # define asmlinkage_protect(n, ret, args...)    do { } while (0)
 #endif