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From: Kevin Wolf <kwolf@redhat.com>
To: qemu-block@nongnu.org
Cc: kwolf@redhat.com,
stefanha@redhat.com,
qemu-devel@nongnu.org
Subject: [PULL 14/58] block: document connection between child roles and
bs->backing/bs->file
Date: Thu, 27 Oct 2022 20:31:02 +0200
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From: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Make the informal rules formal. In further commit we'll add
corresponding assertions.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220726201134.924743-8-vsementsov@yandex-team.ru>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
include/block/block-common.h | 39 ++++++++++++++++++++++++++++++++++++
1 file changed, 39 insertions(+)
diff --git a/include/block/block-common.h b/include/block/block-common.h
index fdb7306e78..fda67a7c38 100644
--- a/include/block/block-common.h
+++ b/include/block/block-common.h
@@ -313,6 +313,45 @@ enum {
*
* At least one of DATA, METADATA, FILTERED, or COW must be set for
* every child.
+ *
+ *
+ * =3D Connection with bs->children, bs->file and bs->backing fields =3D
+ *
+ * 1. Filters
+ *
+ * Filter drivers have drv->is_filter =3D true.
+ *
+ * Filter node has exactly one FILTERED|PRIMARY child, and may have other
+ * children which must not have these bits (one example is the
+ * copy-before-write filter, which also has its target DATA child).
+ *
+ * Filter nodes never have COW children.
+ *
+ * For most filters, the filtered child is linked in bs->file, bs->backing=
is
+ * NULL. For some filters (as an exception), it is the other way around; =
those
+ * drivers will have drv->filtered_child_is_backing set to true (see that
+ * field=E2=80=99s documentation for what drivers this concerns)
+ *
+ * 2. "raw" driver (block/raw-format.c)
+ *
+ * Formally it's not a filter (drv->is_filter =3D false)
+ *
+ * bs->backing is always NULL
+ *
+ * Only has one child, linked in bs->file. Its role is either FILTERED|PRI=
MARY
+ * (like filter) or DATA|PRIMARY depending on options.
+ *
+ * 3. Other drivers
+ *
+ * Don't have any FILTERED children.
+ *
+ * May have at most one COW child. In this case it's linked in bs->backing.
+ * Otherwise bs->backing is NULL. COW child is never PRIMARY.
+ *
+ * May have at most one PRIMARY child. In this case it's linked in bs->fil=
e.
+ * Otherwise bs->file is NULL.
+ *
+ * May also have some other children that don't have the PRIMARY or COW bi=
t set.
*/
enum BdrvChildRoleBits {
/*
--=20
2.37.3