From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50510) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fDsMh-0007OA-Ky for qemu-devel@nongnu.org; Wed, 02 May 2018 10:04:18 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fDsMb-00079y-Io for qemu-devel@nongnu.org; Wed, 02 May 2018 10:04:15 -0400 From: Max Reitz Date: Wed, 2 May 2018 16:03:59 +0200 Message-Id: <20180502140359.18222-1-mreitz@redhat.com> Subject: [Qemu-devel] [PATCH] block: Document BDRV_REQ_WRITE_UNCHANGED support List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-block@nongnu.org Cc: qemu-devel@nongnu.org, Max Reitz , Kevin Wolf , Eric Blake 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 --- I did not include a note on how this might be useful to protocol drivers, because BDRV_REQ_WRITE_UNCHANGED requests will usually not end up on the protocol level anyway. (I suppose it would be possible in theory if a protocol driver reports a certain range as unallocated and the user installs a copy-on-read driver on top, but I think every protocol driver currently reports everything as allocated.) --- include/block/block_int.h | 18 ++++++++++++++++-- 1 file 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. */ -- 2.14.3