block: introduce BDRV_REQ_MAY_UNMAP request flag

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

diff --git a/block-migration.c b/block-migration.c
index 713a8e36e17fe16c9b88dc337a75013d047dc784..fc4ef93ea216cfaa1a7027652e74638d73f224dd 100644 (file)
--- a/block-migration.c
+++ b/block-migration.c
@@ -780,7 +780,8 @@ static int block_load(QEMUFile *f, void *opaque, int version_id)
             }
 
             if (flags & BLK_MIG_FLAG_ZERO_BLOCK) {
-                ret = bdrv_write_zeroes(bs, addr, nr_sectors, 0);
+                ret = bdrv_write_zeroes(bs, addr, nr_sectors,
+                                        BDRV_REQ_MAY_UNMAP);
             } else {
                 buf = g_malloc(BLOCK_SIZE);
                 qemu_get_buffer(f, buf, BLOCK_SIZE);

diff --git a/block.c b/block.c
index e14ba4872045afde7ff62d77ec45e2f1b10afdda..d34e974d07d9a39b1e5c4968e7ad4f15e7113f63 100644 (file)
--- a/block.c
+++ b/block.c
@@ -2818,6 +2818,10 @@ int coroutine_fn bdrv_co_write_zeroes(BlockDriverState *bs,
 {
     trace_bdrv_co_write_zeroes(bs, sector_num, nb_sectors);
 
+    if (!(bs->open_flags & BDRV_O_UNMAP)) {
+        flags &= ~BDRV_REQ_MAY_UNMAP;
+    }
+
     return bdrv_co_do_writev(bs, sector_num, nb_sectors, NULL,
                              BDRV_REQ_ZERO_WRITE | flags);
 }

diff --git a/block/backup.c b/block/backup.c
index 830a179d6d8450ce593970fb560299cdc9c808fa..01985140439ec38efb390de32c525267db854d01 100644 (file)
--- a/block/backup.c
+++ b/block/backup.c
@@ -139,7 +139,7 @@ static int coroutine_fn backup_do_cow(BlockDriverState *bs,
         if (buffer_is_zero(iov.iov_base, iov.iov_len)) {
             ret = bdrv_co_write_zeroes(job->target,
                                        start * BACKUP_SECTORS_PER_CLUSTER,
-                                       n, 0);
+                                       n, BDRV_REQ_MAY_UNMAP);
         } else {
             ret = bdrv_co_writev(job->target,
                                  start * BACKUP_SECTORS_PER_CLUSTER, n,

diff --git a/include/block/block.h b/include/block/block.h
index 8ba9f0c80fb50da9ca1f16d8beed7be8294924ec..1f30a56fb08629cdfdf2af5fceccaab708e1a897 100644 (file)
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -65,6 +65,13 @@ typedef struct BlockDevOps {
 typedef enum {
     BDRV_REQ_COPY_ON_READ = 0x1,
     BDRV_REQ_ZERO_WRITE   = 0x2,
+    /* The BDRV_REQ_MAY_UNMAP flag is used to indicate that the block driver
+     * is allowed to optimize a write zeroes request by unmapping (discarding)
+     * blocks if it is guaranteed that the result will read back as
+     * zeroes. The flag is only passed to the driver if the block device is
+     * opened with BDRV_O_UNMAP.
+     */
+    BDRV_REQ_MAY_UNMAP    = 0x4,
 } BdrvRequestFlags;
 
 #define BDRV_O_RDWR        0x0002