[Qemu-devel] [PATCH 3/6] null: Switch to byte-based read/write

[Eric Blake]

We are gradually moving away from sector-based interfaces, towards
byte-based.  Make the change for the last few sector-based callbacks
in the null-co and null-aio drivers.

Note that since the null driver does nothing on writes, it trivially
supports the BDRV_REQ_FUA flag (all writes have already landed to
the same bit-bucket without needing an extra flush call).  Furthermore,
bdrv_refresh_limits() defaults the block size to 512 for any driver
that does not support coroutines; while this is still correct for the
other aio-based drivers, the null driver does just as well with
byte-based requests, and being explicit means we can avoid cycles
wasted on read-modify-write.

Signed-off-by: Eric Blake <address@hidden>
---
 block/null.c | 66 ++++++++++++++++++++++++++++++++++--------------------------
 1 file changed, 37 insertions(+), 29 deletions(-)

diff --git a/block/null.c b/block/null.c
index 806a8631e4d..2381090dfcf 100644
--- a/block/null.c
+++ b/block/null.c
@@ -93,6 +93,7 @@ static int null_file_open(BlockDriverState *bs, QDict 
*options, int flags,
     }
     s->read_zeroes = qemu_opt_get_bool(opts, NULL_OPT_ZEROES, false);
     qemu_opts_del(opts);
+    bs->supported_write_flags = BDRV_REQ_FUA;
     return ret;
 }

@@ -116,22 +117,22 @@ static coroutine_fn int null_co_common(BlockDriverState 
*bs)
     return 0;
 }

-static coroutine_fn int null_co_readv(BlockDriverState *bs,
-                                      int64_t sector_num, int nb_sectors,
-                                      QEMUIOVector *qiov)
+static coroutine_fn int null_co_preadv(BlockDriverState *bs,
+                                       uint64_t offset, uint64_t bytes,
+                                       QEMUIOVector *qiov, int flags)
 {
     BDRVNullState *s = bs->opaque;

     if (s->read_zeroes) {
-        qemu_iovec_memset(qiov, 0, 0, nb_sectors * BDRV_SECTOR_SIZE);
+        qemu_iovec_memset(qiov, 0, 0, bytes);
     }

     return null_co_common(bs);
 }

-static coroutine_fn int null_co_writev(BlockDriverState *bs,
-                                       int64_t sector_num, int nb_sectors,
-                                       QEMUIOVector *qiov)
+static coroutine_fn int null_co_pwritev(BlockDriverState *bs,
+                                        uint64_t offset, uint64_t bytes,
+                                        QEMUIOVector *qiov, int flags)
 {
     return null_co_common(bs);
 }
@@ -186,26 +187,26 @@ static inline BlockAIOCB 
*null_aio_common(BlockDriverState *bs,
     return &acb->common;
 }

-static BlockAIOCB *null_aio_readv(BlockDriverState *bs,
-                                  int64_t sector_num, QEMUIOVector *qiov,
-                                  int nb_sectors,
-                                  BlockCompletionFunc *cb,
-                                  void *opaque)
-{
-    BDRVNullState *s = bs->opaque;
-
-    if (s->read_zeroes) {
-        qemu_iovec_memset(qiov, 0, 0, nb_sectors * BDRV_SECTOR_SIZE);
-    }
-
-    return null_aio_common(bs, cb, opaque);
-}
-
-static BlockAIOCB *null_aio_writev(BlockDriverState *bs,
-                                   int64_t sector_num, QEMUIOVector *qiov,
-                                   int nb_sectors,
+static BlockAIOCB *null_aio_preadv(BlockDriverState *bs,
+                                   uint64_t offset, uint64_t bytes,
+                                   QEMUIOVector *qiov, int flags,
                                    BlockCompletionFunc *cb,
                                    void *opaque)
+{
+    BDRVNullState *s = bs->opaque;
+
+    if (s->read_zeroes) {
+        qemu_iovec_memset(qiov, 0, 0, bytes);
+    }
+
+    return null_aio_common(bs, cb, opaque);
+}
+
+static BlockAIOCB *null_aio_pwritev(BlockDriverState *bs,
+                                    uint64_t offset, uint64_t bytes,
+                                    QEMUIOVector *qiov, int flags,
+                                    BlockCompletionFunc *cb,
+                                    void *opaque)
 {
     return null_aio_common(bs, cb, opaque);
 }
@@ -256,6 +257,11 @@ static void null_refresh_filename(BlockDriverState *bs, 
QDict *opts)
     bs->full_open_options = opts;
 }

+static void null_refresh_limits(BlockDriverState *bs, Error **errp)
+{
+    bs->bl.request_alignment = 1;
+}
+
 static BlockDriver bdrv_null_co = {
     .format_name            = "null-co",
     .protocol_name          = "null-co",
@@ -266,14 +272,15 @@ static BlockDriver bdrv_null_co = {
     .bdrv_close             = null_close,
     .bdrv_getlength         = null_getlength,

-    .bdrv_co_readv          = null_co_readv,
-    .bdrv_co_writev         = null_co_writev,
+    .bdrv_co_preadv         = null_co_preadv,
+    .bdrv_co_pwritev        = null_co_pwritev,
     .bdrv_co_flush_to_disk  = null_co_flush,
     .bdrv_reopen_prepare    = null_reopen_prepare,

     .bdrv_co_block_status   = null_co_block_status,

     .bdrv_refresh_filename  = null_refresh_filename,
+    .bdrv_refresh_limits    = null_refresh_limits,
 };

 static BlockDriver bdrv_null_aio = {
@@ -286,14 +293,15 @@ static BlockDriver bdrv_null_aio = {
     .bdrv_close             = null_close,
     .bdrv_getlength         = null_getlength,

-    .bdrv_aio_readv         = null_aio_readv,
-    .bdrv_aio_writev        = null_aio_writev,
+    .bdrv_aio_preadv        = null_aio_preadv,
+    .bdrv_aio_pwritev       = null_aio_pwritev,
     .bdrv_aio_flush         = null_aio_flush,
     .bdrv_reopen_prepare    = null_reopen_prepare,

     .bdrv_co_block_status   = null_co_block_status,

     .bdrv_refresh_filename  = null_refresh_filename,
+    .bdrv_refresh_limits    = null_refresh_limits,
 };

 static void bdrv_null_init(void)
-- 
2.14.3