diff --git a/block/io.c b/block/io.c
index 1eb2b2bddc..7e4cb74cf4 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1565,10 +1565,12 @@ static bool bdrv_init_padding(BlockDriverState *bs,
         pad->tail = align - pad->tail;
     }
 
-    if ((!pad->head && !pad->tail) || !bytes) {
+    if (!pad->head && !pad->tail) {
         return false;
     }
 
+    assert(bytes); /* Nothing good in aligning zero-length requests */
+
     sum = pad->head + bytes + pad->tail;
     pad->buf_len = (sum > align && pad->head && pad->tail) ? 2 * align : align;
     pad->buf = qemu_blockalign(bs, pad->buf_len);
@@ -1706,6 +1708,18 @@ int coroutine_fn bdrv_co_preadv_part(BdrvChild *child,
         return ret;
     }
 
+    if (bytes == 0 && !QEMU_IS_ALIGNED(offset, bs->bl.request_alignment)) {
+        /*
+         * Aligning zero request is nonsense. Even if driver has special meaning
+         * of zero-length (like qcow2_co_pwritev_compressed_part), we can't pass
+         * it to driver due to request_alignment.
+         *
+         * Still, no reason to return an error if someone do unaligned
+         * zero-length read occasionally.
+         */
+        return 0;
+    }
+
     bdrv_inc_in_flight(bs);
 
     /* Don't do copy-on-read if we read data before write operation */
@@ -2116,6 +2130,18 @@ int coroutine_fn bdrv_co_pwritev_part(BdrvChild *child,
         return -ENOTSUP;
     }
 
+    if (bytes == 0 && !QEMU_IS_ALIGNED(offset, bs->bl.request_alignment)) {
+        /*
+         * Aligning zero request is nonsense. Even if driver has special meaning
+         * of zero-length (like qcow2_co_pwritev_compressed_part), we can't pass
+         * it to driver due to request_alignment.
+         *
+         * Still, no reason to return an error if someone do unaligned
+         * zero-length write occasionally.
+         */
+        return 0;
+    }
+
     bdrv_inc_in_flight(bs);
     /*
      * Align write if necessary by performing a read-modify-write cycle.
diff --git a/hw/core/machine.c b/hw/core/machine.c
index 3e288bfceb..d8e30e4895 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -148,7 +148,8 @@ GlobalProperty hw_compat_2_5[] = {
 const size_t hw_compat_2_5_len = G_N_ELEMENTS(hw_compat_2_5);
 
 GlobalProperty hw_compat_2_4[] = {
-    { "virtio-blk-device", "scsi", "true" },
+    /* Optional because the 'scsi' property is Linux-only */
+    { "virtio-blk-device", "scsi", "true", .optional = true },
     { "e1000", "extra_mac_registers", "off" },
     { "virtio-pci", "x-disable-pcie", "on" },
     { "virtio-pci", "migrate-extra", "off" },