block: Fragment reads to max transfer length

Drivers should be able to rely on the block layer honoring the
max transfer length, rather than needing to return -EINVAL
(iscsi) or manually fragment things (nbd).  This patch adds
the fragmentation in the block layer, after requests have been
aligned (fragmenting before alignment would lead to multiple
unaligned requests, rather than just the head and tail).

The return value was previously nebulous on success on whether
it was zero or the length read; and fragmenting may introduce
yet other non-zero values if we use the last length read.  But
as at least some callers are sloppy and expect only zero on
success, it is easiest to just guarantee 0.

[Fix uninitialized ret local variable in bdrv_aligned_preadv().
--Stefan]
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-id: 1468607524-19021-2-git-send-email-eblake@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/io.c

@@ -971,21 +971,25 @@ err:
 
 /*
  * Forwards an already correctly aligned request to the BlockDriver. This
- * handles copy on read and zeroing after EOF; any other features must be
- * implemented by the caller.
+ * handles copy on read, zeroing after EOF, and fragmentation of large
+ * reads; any other features must be implemented by the caller.
  */
 static int coroutine_fn bdrv_aligned_preadv(BlockDriverState *bs,
     BdrvTrackedRequest *req, int64_t offset, unsigned int bytes,
     int64_t align, QEMUIOVector *qiov, int flags)
 {
     int64_t total_bytes, max_bytes;
-    int ret;
+    int ret = 0;
+    uint64_t bytes_remaining = bytes;
+    int max_transfer;
 
     assert(is_power_of_2(align));
     assert((offset & (align - 1)) == 0);
     assert((bytes & (align - 1)) == 0);
     assert(!qiov || bytes == qiov->size);
     assert((bs->open_flags & BDRV_O_NO_IO) == 0);
+    max_transfer = QEMU_ALIGN_DOWN(MIN_NON_ZERO(bs->bl.max_transfer, INT_MAX),
+                                   align);
 
     /* TODO: We would need a per-BDS .supported_read_flags and
      * potential fallback support, if we ever implement any read flags
@@ -1024,7 +1028,7 @@ static int coroutine_fn bdrv_aligned_preadv(BlockDriverState *bs,
         }
     }
 
-    /* Forward the request to the BlockDriver */
+    /* Forward the request to the BlockDriver, possibly fragmenting it */
     total_bytes = bdrv_getlength(bs);
     if (total_bytes < 0) {
         ret = total_bytes;
@@ -1032,30 +1036,39 @@ static int coroutine_fn bdrv_aligned_preadv(BlockDriverState *bs,
     }
 
     max_bytes = ROUND_UP(MAX(0, total_bytes - offset), align);
-    if (bytes <= max_bytes) {
+    if (bytes <= max_bytes && bytes <= max_transfer) {
         ret = bdrv_driver_preadv(bs, offset, bytes, qiov, 0);
-    } else if (max_bytes > 0) {
-        QEMUIOVector local_qiov;
+        goto out;
+    }
 
-        qemu_iovec_init(&local_qiov, qiov->niov);
-        qemu_iovec_concat(&local_qiov, qiov, 0, max_bytes);
+    while (bytes_remaining) {
+        int num;
 
-        ret = bdrv_driver_preadv(bs, offset, max_bytes, &local_qiov, 0);
+        if (max_bytes) {
+            QEMUIOVector local_qiov;
 
-        qemu_iovec_destroy(&local_qiov);
-    } else {
-        ret = 0;
-    }
+            num = MIN(bytes_remaining, MIN(max_bytes, max_transfer));
+            assert(num);
+            qemu_iovec_init(&local_qiov, qiov->niov);
+            qemu_iovec_concat(&local_qiov, qiov, bytes - bytes_remaining, num);
 
-    /* Reading beyond end of file is supposed to produce zeroes */
-    if (ret == 0 && total_bytes < offset + bytes) {
-        uint64_t zero_offset = MAX(0, total_bytes - offset);
-        uint64_t zero_bytes = offset + bytes - zero_offset;
-        qemu_iovec_memset(qiov, zero_offset, 0, zero_bytes);
+            ret = bdrv_driver_preadv(bs, offset + bytes - bytes_remaining,
+                                     num, &local_qiov, 0);
+            max_bytes -= num;
+            qemu_iovec_destroy(&local_qiov);
+        } else {
+            num = bytes_remaining;
+            ret = qemu_iovec_memset(qiov, bytes - bytes_remaining, 0,
+                                    bytes_remaining);
+        }
+        if (ret < 0) {
+            goto out;
+        }
+        bytes_remaining -= num;
     }
 
 out:
-    return ret;
+    return ret < 0 ? ret : 0;
 }
 
 /*