[PATCH v2] Improve docs for IOCTL_GNTDEV_MAP_GRANT_REF

[Demi Marie Obenour <demiobenour@xxxxxxxxx>]

The current implementation of gntdev guarantees that the first call to
IOCTL_GNTDEV_MAP_GRANT_REF will set @index to 0.  This is required to
use gntdev for Wayland, which is a future desire of Qubes OS.
Additionally, requesting zero grants results in an error, but this was
not documented either.  Document both of these.

Signed-off-by: Demi Marie Obenour <demiobenour@xxxxxxxxx>
---
 include/uapi/xen/gntdev.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/uapi/xen/gntdev.h b/include/uapi/xen/gntdev.h
index 9ac5515b9bc2..7a7145395c09 100644
--- a/include/uapi/xen/gntdev.h
+++ b/include/uapi/xen/gntdev.h
@@ -47,7 +47,13 @@ struct ioctl_gntdev_grant_ref {
 /*
  * Inserts the grant references into the mapping table of an instance
  * of gntdev. N.B. This does not perform the mapping, which is deferred
- * until mmap() is called with @index as the offset.
+ * until mmap() is called with @index as the offset. @index should be
+ * considered opaque to userspace, with one exception: if no grant
+ * references have ever been inserted into the mapping table of this
+ * instance, @index will be set to 0. This is necessary to use gntdev
+ * with userspace APIs that expect a file descriptor that can be
+ * mmap()'d at offset 0, such as Wayland. If @count is set to 0, this
+ * ioctl will fail.
  */
 #define IOCTL_GNTDEV_MAP_GRANT_REF \
 _IOC(_IOC_NONE, 'G', 0, sizeof(struct ioctl_gntdev_map_grant_ref))
-- 
Sincerely,
Demi Marie Obenour
she/her/hers

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