diff options
| author | Max Reitz <mreitz@redhat.com> | 2018-05-02 16:03:59 +0200 |
|---|---|---|
| committer | Max Reitz <mreitz@redhat.com> | 2018-05-15 16:15:21 +0200 |
| commit | c1e3489dfaa01d215e37b1412759b856c33d44ed (patch) | |
| tree | b86bfa3ae8ba04fd314fc92556151e24112adb30 | |
| parent | 5279b30392da7a3248b320c75f20c61e3a95863c (diff) | |
block: Document BDRV_REQ_WRITE_UNCHANGED support
Add BDRV_REQ_WRITE_UNCHANGED to the list of flags honored during pwrite
and pwrite_zeroes, and also add a note on when you absolutely need to
support it.
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20180502140359.18222-1-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
| -rw-r--r-- | include/block/block_int.h | 18 |
1 files changed, 16 insertions, 2 deletions
diff --git a/include/block/block_int.h b/include/block/block_int.h index e3d6219f4e..76b589da57 100644 --- a/include/block/block_int.h +++ b/include/block/block_int.h @@ -656,10 +656,24 @@ struct BlockDriverState { /* I/O Limits */ BlockLimits bl; - /* Flags honored during pwrite (so far: BDRV_REQ_FUA) */ + /* Flags honored during pwrite (so far: BDRV_REQ_FUA, + * BDRV_REQ_WRITE_UNCHANGED). + * If a driver does not support BDRV_REQ_WRITE_UNCHANGED, those + * writes will be issued as normal writes without the flag set. + * This is important to note for drivers that do not explicitly + * request a WRITE permission for their children and instead take + * the same permissions as their parent did (this is commonly what + * block filters do). Such drivers have to be aware that the + * parent may have taken a WRITE_UNCHANGED permission only and is + * issuing such requests. Drivers either must make sure that + * these requests do not result in plain WRITE accesses (usually + * by supporting BDRV_REQ_WRITE_UNCHANGED, and then forwarding + * every incoming write request as-is, including potentially that + * flag), or they have to explicitly take the WRITE permission for + * their children. */ unsigned int supported_write_flags; /* Flags honored during pwrite_zeroes (so far: BDRV_REQ_FUA, - * BDRV_REQ_MAY_UNMAP) */ + * BDRV_REQ_MAY_UNMAP, BDRV_REQ_WRITE_UNCHANGED) */ unsigned int supported_zero_flags; /* the following member gives a name to every node on the bs graph. */ |