Commit c1e3489d authored by Max Reitz's avatar Max Reitz
Browse files

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: default avatarMax Reitz <mreitz@redhat.com>
Message-id: 20180502140359.18222-1-mreitz@redhat.com
Reviewed-by: default avatarEric Blake <eblake@redhat.com>
Signed-off-by: default avatarMax Reitz <mreitz@redhat.com>
parent 5279b303

include/block/block_int.h

+16 −2
Original line number Diff line number Diff line
@@ -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. */