[Qemu-devel] [PATCH v2 01/20] block: Add .bdrv_co_block_status() callback

[Eric Blake <eblake@redhat.com>]

We are gradually moving away from sector-based interfaces, towards
byte-based. Now that the block layer exposes byte-based allocation,
it's time to tackle the drivers.  Add a new callback that operates
on as small as byte boundaries. Subsequent patches will then update
individual drivers, then finally remove .bdrv_co_get_block_status().
The old code now uses a goto in order to minimize churn at that later
removal.

The new code also passes through the 'mapping' hint, which will
allow subsequent patches to further optimize callers that only care
about how much of the image is allocated, rather than which offsets
the allocation actually maps to.

Note that most drivers give sector-aligned answers, except at
end-of-file, even when request_alignment is smaller than a sector.
However, bdrv_getlength() is sector-aligned (even though it gives a
byte answer), often by exceeding the actual file size.  If we were to
give back strict results, at least file-posix.c would report a
transition from DATA to HOLE at the end of a file even in the middle
of a sector, which can throw off callers; so we intentionally lie and
state that any partial sector at the end of a file has the same
status for the entire sector.

Signed-off-by: Eric Blake <eblake@redhat.com>

---
v2: improve alignment handling, ensure all iotests still pass
---
 include/block/block_int.h | 11 ++++++++---
 block/io.c                | 27 ++++++++++++++++++++++++---
 2 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/include/block/block_int.h b/include/block/block_int.h
index f2c1da7..aeb2bf8 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -176,13 +176,18 @@ struct BlockDriver {
      * bdrv_is_allocated[_above].  The driver should answer only
      * according to the current layer, and should not set
      * BDRV_BLOCK_ALLOCATED, but may set BDRV_BLOCK_RAW.  See block.h
-     * for the meaning of _DATA, _ZERO, and _OFFSET_VALID.  The block
-     * layer guarantees input aligned to request_alignment, as well as
-     * non-NULL pnum and file.
+     * for the meaning of _DATA, _ZERO, and _OFFSET_VALID.  As a hint,
+     * the flag mapping is true if the caller cares more about precise
+     * mappings (favor _OFFSET_VALID) or false for overall allocation
+     * (favor larger *pnum).  The block layer guarantees input aligned
+     * to request_alignment, as well as non-NULL pnum and file.
      */
     int64_t coroutine_fn (*bdrv_co_get_block_status)(BlockDriverState *bs,
         int64_t sector_num, int nb_sectors, int *pnum,
         BlockDriverState **file);
+    int64_t coroutine_fn (*bdrv_co_block_status)(BlockDriverState *bd,
+        bool mapping, int64_t offset, int64_t bytes, int64_t *pnum,
+        BlockDriverState **file);

     /*
      * Invalidate any cached meta-data.
diff --git a/block/io.c b/block/io.c
index 0e4a3e6..899a7b3 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1770,7 +1770,7 @@ static int64_t coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
         bytes = n;
     }

-    if (!bs->drv->bdrv_co_get_block_status) {
+    if (!bs->drv->bdrv_co_get_block_status && !bs->drv->bdrv_co_block_status) {
         *pnum = bytes;
         ret = BDRV_BLOCK_DATA | BDRV_BLOCK_ALLOCATED;
         if (offset + bytes == total_size) {
@@ -1790,11 +1790,14 @@ static int64_t coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
     bdrv_inc_in_flight(bs);

     /* Round out to request_alignment boundaries */
-    align = MAX(bs->bl.request_alignment, BDRV_SECTOR_SIZE);
+    align = bs->bl.request_alignment;
+    if (bs->drv->bdrv_co_get_block_status && align < BDRV_SECTOR_SIZE) {
+        align = BDRV_SECTOR_SIZE;
+    }
     aligned_offset = QEMU_ALIGN_DOWN(offset, align);
     aligned_bytes = ROUND_UP(offset + bytes, align) - aligned_offset;

-    {
+    if (bs->drv->bdrv_co_get_block_status) {
         int count; /* sectors */

         assert(QEMU_IS_ALIGNED(aligned_offset | aligned_bytes,
@@ -1808,8 +1811,26 @@ static int64_t coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
             goto out;
         }
         *pnum = count * BDRV_SECTOR_SIZE;
+        goto refine;
     }

+    ret = bs->drv->bdrv_co_block_status(bs, mapping, aligned_offset,
+                                        aligned_bytes, pnum, &local_file);
+    if (ret < 0) {
+        *pnum = 0;
+        goto out;
+    }
+
+    /*
+     * total_size is always sector-aligned, by sometimes exceeding actual
+     * file size. Expand pnum if it lands mid-sector due to end-of-file.
+     */
+    if (QEMU_ALIGN_UP(*pnum + aligned_offset,
+                      BDRV_SECTOR_SIZE) == total_size) {
+        *pnum = total_size - aligned_offset;
+    }
+
+ refine:
     /* Clamp pnum and ret to original request */
     assert(QEMU_IS_ALIGNED(*pnum, align));
     *pnum -= offset - aligned_offset;
-- 
2.9.4