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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>
This commit is contained in:
Demi Marie Obenour 2022-01-31 12:23:07 -05:00 committed by Juergen Gross
parent 4564661af6
commit 164666fa66
1 changed files with 7 additions and 1 deletions
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8
include/uapi/xen/gntdev.h
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@ -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))