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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>
master
Max Reitz 2018-05-02 16:03:59 +02:00
parent 5279b30392
commit c1e3489dfa
1 changed files with 16 additions and 2 deletions
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18
include/block/block_int.h
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@ -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. */