qcow2: Handle dependencies earlier

Handling overlapping allocations isn't just a detail of cluster
allocation. It is rather one of three ways to get the host cluster
offset for a write request:

1. If a request overlaps an in-flight allocations, the cluster offset
   can be taken from there (this is what handle_dependencies will evolve
   into) or the request must just wait until the allocation has
   completed. Accessing the L2 is not valid in this case, it has
   outdated information.

2. Outside overlapping areas, check the clusters that can be written to
   as they are, with no COW involved.

3. If a COW is required, allocate new clusters

Changing the code to reflect this doesn't change the behaviour because
overlaps cannot exist for clusters that are kept in step 2. It does
however make it easier for later patches to work on clusters that belong
to an allocation that is still in flight.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/block/qcow2.h b/block/qcow2.h
index e4b5e11a91..0940b1b3e8 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -277,6 +277,11 @@ static inline int size_to_l1(BDRVQcowState *s, int64_t size)
     return (size + (1ULL << shift) - 1) >> shift;
 }
 
+static inline int offset_to_l2_index(BDRVQcowState *s, int64_t offset)
+{
+    return (offset >> s->cluster_bits) & (s->l2_size - 1);
+}
+
 static inline int64_t align_offset(int64_t offset, int n)
 {
     offset = (offset + n - 1) & ~(n - 1);