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[Xen-devel] [PATCH XEN v6 27/32] tools/libs/call: Describe return values and error semantics for xencall*

This behaviour has been confirmed by inspection on:

 - Linux
 - 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 FreeBSD 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@xxxxxxxxxx>
Cc: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
v6: New patch
 tools/libs/call/include/xencall.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/tools/libs/call/include/xencall.h 
index 0d91aa8..3f325f0 100644
--- 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,

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