@@ -777,20 +777,38 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta *m)
Qcow2COWRegion *start = &m->cow_start;
Qcow2COWRegion *end = &m->cow_end;
unsigned buffer_size;
+ unsigned data_bytes = end->offset - (start->offset + start->nb_bytes);
+ bool merge_reads;
uint8_t *start_buffer, *end_buffer;
int ret;
assert(start->nb_bytes <= UINT_MAX - end->nb_bytes);
+ assert(start->nb_bytes + end->nb_bytes <= UINT_MAX - data_bytes);
+ assert(start->offset + start->nb_bytes <= end->offset);
if (start->nb_bytes == 0 && end->nb_bytes == 0) {
return 0;
}
- /* Reserve a buffer large enough to store the data from both the
- * start and end COW regions. Add some padding in the middle if
- * necessary to make sure that the end region is optimally aligned */
- buffer_size = QEMU_ALIGN_UP(start->nb_bytes, bdrv_opt_mem_align(bs)) +
- end->nb_bytes;
+ /* If we have to read both the start and end COW regions and the
+ * middle region is not too large then perform just one read
+ * operation */
+ merge_reads = start->nb_bytes && end->nb_bytes && data_bytes <= 16384;
+ if (merge_reads) {
+ buffer_size = start->nb_bytes + data_bytes + end->nb_bytes;
+ } else {
+ /* If we have to do two reads, add some padding in the middle
+ * if necessary to make sure that the end region is optimally
+ * aligned. */
+ size_t align = bdrv_opt_mem_align(bs);
+ assert(align > 0 && align <= UINT_MAX);
+ assert(QEMU_ALIGN_UP(start->nb_bytes, align) <=
+ UINT_MAX - end->nb_bytes);
+ buffer_size = QEMU_ALIGN_UP(start->nb_bytes, align) + end->nb_bytes;
+ }
+
+ /* Reserve a buffer large enough to store all the data that we're
+ * going to read */
start_buffer = qemu_try_blockalign(bs, buffer_size);
if (start_buffer == NULL) {
return -ENOMEM;
@@ -799,15 +817,22 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta *m)
end_buffer = start_buffer + buffer_size - end->nb_bytes;
qemu_co_mutex_unlock(&s->lock);
- /* First we read the existing data from both COW regions */
- ret = do_perform_cow_read(bs, m->offset, start->offset,
- start_buffer, start->nb_bytes);
- if (ret < 0) {
- goto fail;
- }
+ /* First we read the existing data from both COW regions. We
+ * either read the whole region in one go, or the start and end
+ * regions separately. */
+ if (merge_reads) {
+ ret = do_perform_cow_read(bs, m->offset, start->offset,
+ start_buffer, buffer_size);
+ } else {
+ ret = do_perform_cow_read(bs, m->offset, start->offset,
+ start_buffer, start->nb_bytes);
+ if (ret < 0) {
+ goto fail;
+ }
- ret = do_perform_cow_read(bs, m->offset, end->offset,
- end_buffer, end->nb_bytes);
+ ret = do_perform_cow_read(bs, m->offset, end->offset,
+ end_buffer, end->nb_bytes);
+ }
if (ret < 0) {
goto fail;
}