util/iov: improve qemu_iovec_is_zero

We'll need to check a part of qiov soon, so implement it now.

Optimization with align down to 4 * sizeof(long) is dropped due to:
1. It is strange: it aligns length of the buffer, but where is a
   guarantee that buffer pointer is aligned itself?
2. buffer_is_zero() is a better place for optimizations and it has
   them.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190604161514.262241-3-vsementsov@virtuozzo.com
Message-Id: <20190604161514.262241-3-vsementsov@virtuozzo.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

3 files changed, 21 insertions, 14 deletions

diff --git a/block/io.c b/block/io.c
index 56bbf195bb..f656fb2dce 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1722,7 +1722,7 @@ static int coroutine_fn bdrv_aligned_pwritev(BdrvChild *child,
 
     if (!ret && bs->detect_zeroes != BLOCKDEV_DETECT_ZEROES_OPTIONS_OFF &&
         !(flags & BDRV_REQ_ZERO_WRITE) && drv->bdrv_co_pwrite_zeroes &&
-        qemu_iovec_is_zero(qiov)) {
+        qemu_iovec_is_zero(qiov, 0, qiov->size)) {
         flags |= BDRV_REQ_ZERO_WRITE;
         if (bs->detect_zeroes == BLOCKDEV_DETECT_ZEROES_OPTIONS_UNMAP) {
             flags |= BDRV_REQ_MAY_UNMAP;
diff --git a/include/qemu/iov.h b/include/qemu/iov.h
index f3787a0cf7..29957c8a72 100644
--- a/include/qemu/iov.h
+++ b/include/qemu/iov.h
@@ -212,7 +212,7 @@ void qemu_iovec_concat(QEMUIOVector *dst,
 size_t qemu_iovec_concat_iov(QEMUIOVector *dst,
                              struct iovec *src_iov, unsigned int src_cnt,
                              size_t soffset, size_t sbytes);
-bool qemu_iovec_is_zero(QEMUIOVector *qiov);
+bool qemu_iovec_is_zero(QEMUIOVector *qiov, size_t qiov_offeset, size_t bytes);
 void qemu_iovec_destroy(QEMUIOVector *qiov);
 void qemu_iovec_reset(QEMUIOVector *qiov);
 size_t qemu_iovec_to_buf(QEMUIOVector *qiov, size_t offset,
diff --git a/util/iov.c b/util/iov.c
index 366ff9cdd1..9ac0261853 100644
--- a/util/iov.c
+++ b/util/iov.c
@@ -451,23 +451,30 @@ void qemu_iovec_init_extended(
 }
 
 /*
- * Check if the contents of the iovecs are all zero
+ * Check if the contents of subrange of qiov data is all zeroes.
  */
-bool qemu_iovec_is_zero(QEMUIOVector *qiov)
+bool qemu_iovec_is_zero(QEMUIOVector *qiov, size_t offset, size_t bytes)
 {
-    int i;
-    for (i = 0; i < qiov->niov; i++) {
-        size_t offs = QEMU_ALIGN_DOWN(qiov->iov[i].iov_len, 4 * sizeof(long));
-        uint8_t *ptr = qiov->iov[i].iov_base;
-        if (offs && !buffer_is_zero(qiov->iov[i].iov_base, offs)) {
+    struct iovec *iov;
+    size_t current_offset;
+
+    assert(offset + bytes <= qiov->size);
+
+    iov = iov_skip_offset(qiov->iov, offset, &current_offset);
+
+    while (bytes) {
+        uint8_t *base = (uint8_t *)iov->iov_base + current_offset;
+        size_t len = MIN(iov->iov_len - current_offset, bytes);
+
+        if (!buffer_is_zero(base, len)) {
             return false;
         }
-        for (; offs < qiov->iov[i].iov_len; offs++) {
-            if (ptr[offs]) {
-                return false;
-            }
-        }
+
+        current_offset = 0;
+        bytes -= len;
+        iov++;
     }
+
     return true;
 }