From 24b7c538fea86b598e2a335f4805a0ab50a30e98 Mon Sep 17 00:00:00 2001 From: Max Reitz Date: Sat, 21 Apr 2018 15:29:22 +0200 Subject: block: BLK_PERM_WRITE includes ..._UNCHANGED Currently we never actually check whether the WRITE_UNCHANGED permission has been taken for unchanging writes. But the one check that is commented out checks both WRITE and WRITE_UNCHANGED; and considering that WRITE_UNCHANGED is already documented as being weaker than WRITE, we should probably explicitly document WRITE to include WRITE_UNCHANGED. Signed-off-by: Max Reitz Reviewed-by: Stefan Hajnoczi Reviewed-by: Alberto Garcia Message-id: 20180421132929.21610-3-mreitz@redhat.com Reviewed-by: Kevin Wolf Signed-off-by: Max Reitz --- include/block/block.h | 3 +++ 1 file changed, 3 insertions(+) (limited to 'include') diff --git a/include/block/block.h b/include/block/block.h index cdec3639a3..397b5e8d44 100644 --- a/include/block/block.h +++ b/include/block/block.h @@ -205,6 +205,9 @@ enum { * This permission (which is weaker than BLK_PERM_WRITE) is both enough and * required for writes to the block node when the caller promises that * the visible disk content doesn't change. + * + * As the BLK_PERM_WRITE permission is strictly stronger, either is + * sufficient to perform an unchanging write. */ BLK_PERM_WRITE_UNCHANGED = 0x04, -- cgit 1.4.1 From c6035964f8316b504060618d05b5dd434f18595b Mon Sep 17 00:00:00 2001 From: Max Reitz Date: Sat, 21 Apr 2018 15:29:23 +0200 Subject: block: Add BDRV_REQ_WRITE_UNCHANGED flag This flag signifies that a write request will not change the visible disk content. With this flag set, it is sufficient to have the BLK_PERM_WRITE_UNCHANGED permission instead of BLK_PERM_WRITE. Signed-off-by: Max Reitz Reviewed-by: Stefan Hajnoczi Reviewed-by: Alberto Garcia Message-id: 20180421132929.21610-4-mreitz@redhat.com Reviewed-by: Kevin Wolf Signed-off-by: Max Reitz --- block/io.c | 6 +++++- include/block/block.h | 6 +++++- 2 files changed, 10 insertions(+), 2 deletions(-) (limited to 'include') diff --git a/block/io.c b/block/io.c index 4fad5ac2fe..9e8449e795 100644 --- a/block/io.c +++ b/block/io.c @@ -1504,7 +1504,11 @@ static int coroutine_fn bdrv_aligned_pwritev(BdrvChild *child, assert(!waited || !req->serialising); assert(req->overlap_offset <= offset); assert(offset + bytes <= req->overlap_offset + req->overlap_bytes); - assert(child->perm & BLK_PERM_WRITE); + if (flags & BDRV_REQ_WRITE_UNCHANGED) { + assert(child->perm & (BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE)); + } else { + assert(child->perm & BLK_PERM_WRITE); + } assert(end_sector <= bs->total_sectors || child->perm & BLK_PERM_RESIZE); ret = notifier_with_return_list_notify(&bs->before_write_notifiers, req); diff --git a/include/block/block.h b/include/block/block.h index 397b5e8d44..3894edda9d 100644 --- a/include/block/block.h +++ b/include/block/block.h @@ -54,8 +54,12 @@ typedef enum { BDRV_REQ_FUA = 0x10, BDRV_REQ_WRITE_COMPRESSED = 0x20, + /* Signifies that this write request will not change the visible disk + * content. */ + BDRV_REQ_WRITE_UNCHANGED = 0x40, + /* Mask of valid flags */ - BDRV_REQ_MASK = 0x3f, + BDRV_REQ_MASK = 0x7f, } BdrvRequestFlags; typedef struct BlockSizes { -- cgit 1.4.1 From c1e3489dfaa01d215e37b1412759b856c33d44ed Mon Sep 17 00:00:00 2001 From: Max Reitz Date: Wed, 2 May 2018 16:03:59 +0200 Subject: 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 Message-id: 20180502140359.18222-1-mreitz@redhat.com Reviewed-by: Eric Blake Signed-off-by: Max Reitz --- include/block/block_int.h | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) (limited to 'include') 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. */ -- cgit 1.4.1