[PATCHv2,2/7] allow qemu_iovec_from_buffer() to specify offset from which to start copying

diff --git a/block.c b/block.c
index 52ffe14..cbbedfa 100644
--- a/block.c
+++ b/block.c
@@ -1692,7 +1692,7 @@  static int coroutine_fn bdrv_co_do_copy_on_readv(BlockDriverState *bs,
     }
 
     skip_bytes = (sector_num - cluster_sector_num) * BDRV_SECTOR_SIZE;
-    qemu_iovec_from_buffer(qiov, bounce_buffer + skip_bytes,
+    qemu_iovec_from_buffer(qiov, 0, bounce_buffer + skip_bytes,
                            nb_sectors * BDRV_SECTOR_SIZE);
 
 err:
@@ -3240,7 +3240,7 @@  static void bdrv_aio_bh_cb(void *opaque)
     BlockDriverAIOCBSync *acb = opaque;
 
     if (!acb->is_write)
-        qemu_iovec_from_buffer(acb->qiov, acb->bounce, acb->qiov->size);
+        qemu_iovec_from_buffer(acb->qiov, 0, acb->bounce, acb->qiov->size);
     qemu_vfree(acb->bounce);
     acb->common.cb(acb->common.opaque, acb->ret);
     qemu_bh_delete(acb->bh);
diff --git a/block/curl.c b/block/curl.c
index e9102e3..e772912 100644
--- a/block/curl.c
+++ b/block/curl.c
@@ -142,7 +142,7 @@  static size_t curl_read_cb(void *ptr, size_t size, size_t nmemb, void *opaque)
             continue;
 
         if ((s->buf_off >= acb->end)) {
-            qemu_iovec_from_buffer(acb->qiov, s->orig_buf + acb->start,
+            qemu_iovec_from_buffer(acb->qiov, 0, s->orig_buf + acb->start,
                                    acb->end - acb->start);
             acb->common.cb(acb->common.opaque, 0);
             qemu_aio_release(acb);
@@ -178,7 +178,7 @@  static int curl_find_buf(BDRVCURLState *s, size_t start, size_t len,
         {
             char *buf = state->orig_buf + (start - state->buf_start);
 
-            qemu_iovec_from_buffer(acb->qiov, buf, len);
+            qemu_iovec_from_buffer(acb->qiov, 0, buf, len);
             acb->common.cb(acb->common.opaque, 0);
 
             return FIND_RET_OK;
diff --git a/block/qcow.c b/block/qcow.c
index b1cfe1f..f95b93f 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -540,7 +540,7 @@  done:
     qemu_co_mutex_unlock(&s->lock);
 
     if (qiov->niov > 1) {
-        qemu_iovec_from_buffer(qiov, orig_buf, qiov->size);
+        qemu_iovec_from_buffer(qiov, 0, orig_buf, qiov->size);
         qemu_vfree(orig_buf);
     }
 
diff --git a/block/qcow2.c b/block/qcow2.c
index 6d11bc0..668e2e2 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -475,7 +475,7 @@  static coroutine_fn int qcow2_co_readv(BlockDriverState *bs, int64_t sector_num,
                 goto fail;
             }
 
-            qemu_iovec_from_buffer(&hd_qiov,
+            qemu_iovec_from_buffer(&hd_qiov, 0,
                 s->cluster_cache + index_in_cluster * 512,
                 512 * cur_nr_sectors);
         } else {
@@ -516,7 +516,7 @@  static coroutine_fn int qcow2_co_readv(BlockDriverState *bs, int64_t sector_num,
                 qemu_iovec_reset(&hd_qiov);
                 qemu_iovec_copy(&hd_qiov, qiov, bytes_done,
                     cur_nr_sectors * 512);
-                qemu_iovec_from_buffer(&hd_qiov, cluster_data,
+                qemu_iovec_from_buffer(&hd_qiov, 0, cluster_data,
                     512 * cur_nr_sectors);
             }
         }
diff --git a/block/vdi.c b/block/vdi.c
index 6a0011f..db0650e 100644
--- a/block/vdi.c
+++ b/block/vdi.c
@@ -635,7 +635,7 @@  static void vdi_aio_read_cb(void *opaque, int ret)
     return;
 done:
     if (acb->qiov->niov > 1) {
-        qemu_iovec_from_buffer(acb->qiov, acb->orig_buf, acb->qiov->size);
+        qemu_iovec_from_buffer(acb->qiov, 0, acb->orig_buf, acb->qiov->size);
         qemu_vfree(acb->orig_buf);
     }
     acb->common.cb(acb->common.opaque, ret);
diff --git a/cutils.c b/cutils.c
index 9451c86..5353f89 100644
--- a/cutils.c
+++ b/cutils.c
@@ -244,19 +244,26 @@  void qemu_iovec_to_buffer(QEMUIOVector *qiov, void *buf)
     }
 }
 
-void qemu_iovec_from_buffer(QEMUIOVector *qiov, const void *buf, size_t count)
+void qemu_iovec_from_buffer(QEMUIOVector *qiov, size_t offset,
+			    const void *buf, size_t bytes)
 {
-    const uint8_t *p = (const uint8_t *)buf;
-    size_t copy;
+    const struct iovec *iov = qiov->iov;
     int i;
+    assert(qiov->size >= offset);
+    assert(qiov->size - offset >= bytes);
 
-    for (i = 0; i < qiov->niov && count; ++i) {
-        copy = count;
-        if (copy > qiov->iov[i].iov_len)
-            copy = qiov->iov[i].iov_len;
-        memcpy(qiov->iov[i].iov_base, p, copy);
-        p     += copy;
-        count -= copy;
+    /* first skip initial full-sized elements */
+    for(i = 0; offset >= iov[i].iov_len; ++i) {
+	offset -= iov[i].iov_len;
+    }
+    /* skip/copy partial element and copy the rest */
+    while(bytes) {
+	size_t n = MIN(bytes, iov[i].iov_len - offset);
+	memcpy((char*)iov[i].iov_base + offset, buf, n);
+	bytes -= n;
+        buf = (const char*)buf + n;
+	++i;
+	offset = 0;
     }
 }
 
diff --git a/qemu-common.h b/qemu-common.h
index a1ff126..249f70d 100644
--- a/qemu-common.h
+++ b/qemu-common.h
@@ -339,7 +339,8 @@  void qemu_iovec_concat(QEMUIOVector *dst, QEMUIOVector *src, size_t size);
 void qemu_iovec_destroy(QEMUIOVector *qiov);
 void qemu_iovec_reset(QEMUIOVector *qiov);
 void qemu_iovec_to_buffer(QEMUIOVector *qiov, void *buf);
-void qemu_iovec_from_buffer(QEMUIOVector *qiov, const void *buf, size_t count);
+void qemu_iovec_from_buffer(QEMUIOVector *qiov, size_t offset,
+			    const void *buf, size_t bytes);
 void qemu_iovec_memset(QEMUIOVector *qiov, size_t offset, int c, size_t bytes);
 
 bool buffer_is_zero(const void *buf, size_t len);