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Improve docs for IOCTL_GNTDEV_MAP_GRANT_REF

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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@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/f66c5a4e-2034-00b5-a635-6983bd999c07@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>

authored by

Demi Marie Obenour and committed by
Juergen Gross 164666fa 4564661a

+7 -1
+7 -1
include/uapi/xen/gntdev.h
··· 47 47 /* 48 48 * Inserts the grant references into the mapping table of an instance 49 49 * of gntdev. N.B. This does not perform the mapping, which is deferred 50 - * until mmap() is called with @index as the offset. 50 + * until mmap() is called with @index as the offset. @index should be 51 + * considered opaque to userspace, with one exception: if no grant 52 + * references have ever been inserted into the mapping table of this 53 + * instance, @index will be set to 0. This is necessary to use gntdev 54 + * with userspace APIs that expect a file descriptor that can be 55 + * mmap()'d at offset 0, such as Wayland. If @count is set to 0, this 56 + * ioctl will fail. 51 57 */ 52 58 #define IOCTL_GNTDEV_MAP_GRANT_REF \ 53 59 _IOC(_IOC_NONE, 'G', 0, sizeof(struct ioctl_gntdev_map_grant_ref))