tools/libs/call: Describe return values and error semantics for xencall*

This behaviour has been confirmed by inspection on:

 - Linux
 - NetBSD & FreeBSD (NB: hcall->retval is the hypercall return value
   only for values >= 0. For negative values the underlying privcmd
   driver translates the value from Xen to {Net,Free}BSD errno space
   and returns it as the result of the ioctl, which becomes
   ret=-1/errno=EFOO in userspace)
 - MiniOS (which takes care of errno in this library)

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Roger Pau Monné <roger.pau@citrix.com>

diff --git a/tools/libs/call/include/xencall.h b/tools/libs/call/include/xencall.h
index 0d91aa8414eab41e6871f5d9398267b8d1b4d49f..3f325f0a5ed58761f864c13b23e028e1d849649c 100644 (file)
--- a/tools/libs/call/include/xencall.h
+++ b/tools/libs/call/include/xencall.h
@@ -46,6 +46,21 @@ int xencall_close(xencall_handle *xcall);
 
 /*
  * Call hypercalls with varying numbers of arguments.
+ *
+ * On success the return value of the hypercall is the return value of
+ * the xencall function.  On error these functions set errno and
+ * return -1.
+ *
+ * The errno values will be either:
+ * - The Xen hypercall error return (from xen/include/public/errno.h)
+ *   translated into the corresponding local value for that POSIX error.
+ * - An errno value produced by the OS driver or the library
+ *   implementation. Such values may be defined by POSIX or by the OS.
+ *
+ * Note that under some circumstances it will not be possible to tell
+ * whether an error came from Xen or from the OS/library.
+ *
+ * These functions never log.
  */
 int xencall0(xencall_handle *xcall, unsigned int op);
 int xencall1(xencall_handle *xcall, unsigned int op,