[PATCH 1/4 V2] block: add SCSI COMPARE_AND_WRITE support

[Yaowei Bai <baiyaowei@cmss.chinamobile.com>]

Some storages(i.e. librbd) already have interfaces to handle some SCSI
commands directly. This patch adds COMPARE_AND_WRITE command support
through the write path in the block io layer by introducing a new element
BDRV_REQ_COMPARE_AND_WRITE into BdrvRequestFlags which indicates a
COMPARE_AND_WRITE request. In this way we could easily extend to other
SCSI commands support like WRITE_SAME in the future.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
---
 block/io.c                | 20 ++++++++++++++++++++
 include/block/block.h     |  5 +++--
 include/block/block_int.h |  3 +++
 3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/block/io.c b/block/io.c
index f75777f..3507d71 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1186,6 +1186,26 @@ static int coroutine_fn bdrv_driver_pwritev(BlockDriverState *bs,
         goto emulate_flags;
     }
 
+    if (drv->bdrv_aio_compare_and_write &&
+                              (flags & BDRV_REQ_COMPARE_AND_WRITE)) {
+        BlockAIOCB *acb;
+        CoroutineIOCompletion co = {
+            .coroutine = qemu_coroutine_self(),
+        };
+
+        acb = drv->bdrv_aio_compare_and_write(bs, offset, bytes, qiov,
+                                    flags & bs->supported_write_flags,
+                                    bdrv_co_io_em_complete, &co);
+        flags &= ~bs->supported_write_flags;
+        if (acb == NULL) {
+            ret = -EIO;
+        } else {
+            qemu_coroutine_yield();
+            ret = co.ret;
+        }
+        goto emulate_flags;
+    }
+
     if (drv->bdrv_aio_pwritev) {
         BlockAIOCB *acb;
         CoroutineIOCompletion co = {
diff --git a/include/block/block.h b/include/block/block.h
index 1df9848..f71aa4f 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -92,9 +92,10 @@ typedef enum {
      * on read request and means that caller doesn't really need data to be
      * written to qiov parameter which may be NULL.
      */
-    BDRV_REQ_PREFETCH  = 0x200,
+    BDRV_REQ_PREFETCH           = 0x200,
+    BDRV_REQ_COMPARE_AND_WRITE  = 0x400,
     /* Mask of valid flags */
-    BDRV_REQ_MASK               = 0x3ff,
+    BDRV_REQ_MASK               = 0x7ff,
 } BdrvRequestFlags;
 
 typedef struct BlockSizes {
diff --git a/include/block/block_int.h b/include/block/block_int.h
index dd033d0..96096e0 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -189,6 +189,9 @@ struct BlockDriver {
     BlockAIOCB *(*bdrv_aio_pdiscard)(BlockDriverState *bs,
         int64_t offset, int bytes,
         BlockCompletionFunc *cb, void *opaque);
+    BlockAIOCB *(*bdrv_aio_compare_and_write)(BlockDriverState *bs,
+        uint64_t offset, uint64_t bytes, QEMUIOVector *qiov, int flags,
+        BlockCompletionFunc *cb, void *opaque);
 
     int coroutine_fn (*bdrv_co_readv)(BlockDriverState *bs,
         int64_t sector_num, int nb_sectors, QEMUIOVector *qiov);
-- 
1.8.3.1