block: include supported_read_flags into BDS structure

Add the new member supported_read_flags to the BlockDriverState
structure. It will control the flags set for copy-on-read operations.
Make the block generic layer evaluate supported read flags before they
go to a block driver.

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
 [vsementsov: use assert instead of abort]
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201216061703.70908-8-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/io.c

@@ -1453,6 +1453,9 @@ static int coroutine_fn bdrv_aligned_preadv(BdrvChild *child,
     if (flags & BDRV_REQ_COPY_ON_READ) {
         int64_t pnum;
 
+        /* The flag BDRV_REQ_COPY_ON_READ has reached its addressee */
+        flags &= ~BDRV_REQ_COPY_ON_READ;
+
         ret = bdrv_is_allocated(bs, offset, bytes, &pnum);
         if (ret < 0) {
             goto out;
@@ -1474,9 +1477,11 @@ static int coroutine_fn bdrv_aligned_preadv(BdrvChild *child,
         goto out;
     }
 
+    assert(!(flags & ~bs->supported_read_flags));
+
     max_bytes = ROUND_UP(MAX(0, total_bytes - offset), align);
     if (bytes <= max_bytes && bytes <= max_transfer) {
-        ret = bdrv_driver_preadv(bs, offset, bytes, qiov, qiov_offset, 0);
+        ret = bdrv_driver_preadv(bs, offset, bytes, qiov, qiov_offset, flags);
         goto out;
     }
 
@@ -1489,7 +1494,8 @@ static int coroutine_fn bdrv_aligned_preadv(BdrvChild *child,
 
             ret = bdrv_driver_preadv(bs, offset + bytes - bytes_remaining,
                                      num, qiov,
-                                     qiov_offset + bytes - bytes_remaining, 0);
+                                     qiov_offset + bytes - bytes_remaining,
+                                     flags);
             max_bytes -= num;
         } else {
             num = bytes_remaining;

include/block/block_int.h

@@ -881,6 +881,10 @@ struct BlockDriverState {
     /* I/O Limits */
     BlockLimits bl;
 
+    /*
+     * Flags honored during pread
+     */
+    unsigned int supported_read_flags;
     /* Flags honored during pwrite (so far: BDRV_REQ_FUA,
      * BDRV_REQ_WRITE_UNCHANGED).
      * If a driver does not support BDRV_REQ_WRITE_UNCHANGED, those