[RFC PATCH 3/3] iov, vu_common: Make iov_from_buf() fill destination iov entirely

[Stefano Brivio <sbrivio@redhat.com>]

...and, for consistency, rename 'bytes' to 'copy' in iov_to_buf().

Two commits ago, I changed vhost-user functions to use iov_from_buf()
to copy only up to the size of source buffers, instead of using the
size of the destination vhost-user buffers.

This change pads the rest with zeroes, which is not strictly needed,
but looks definitely cleaner.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 iov.c       | 54 ++++++++++++++++++++++++++++++++++-------------------
 iov.h       |  4 ++--
 vu_common.c |  2 +-
 3 files changed, 38 insertions(+), 22 deletions(-)

diff --git a/iov.c b/iov.c
index dc1b6b1..557be55 100644
--- a/iov.c
+++ b/iov.c
@@ -59,35 +59,51 @@ size_t iov_skip_bytes(const struct iovec *iov, size_t n,
  * @iov_cnt:	Number of elements in the iovec array
  * @offset:	Destination offset in iovec array
  * @buf:	Source buffer
- * @bytes:	Bytes to copy
+ * @copy:	Bytes to copy
+ * @fill:	Bytes to zero-fill after copied bytes
  *
- * Return: number of bytes copied
+ * Return: number of bytes filled, with data or zeroes
  */
 size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
-		    size_t offset, const void *buf, size_t bytes)
+		    size_t offset, const void *buf, size_t copy, size_t fill)
 {
+	size_t copied, filled;
 	unsigned int i;
-	size_t copied;
 
-	if (__builtin_constant_p(bytes) && iov_cnt &&
-		offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
-		memcpy((char *)iov[0].iov_base + offset, buf, bytes);
+	if (__builtin_constant_p(copy) && iov_cnt &&
+		offset <= iov[0].iov_len &&
+		(copy + fill) <= iov[0].iov_len - offset) {
+		memcpy((char *)iov[0].iov_base + offset, buf, copy);
+		memset((char *)iov[0].iov_base + offset + copy, 0, fill);
 
-		return bytes;
+		return copy + fill;
 	}
 
 	i = iov_skip_bytes(iov, iov_cnt, offset, &offset);
 
-	for (copied = 0; copied < bytes && i < iov_cnt; i++) {
-		size_t len = MIN(iov[i].iov_len - offset, bytes - copied);
+	for (copied = 0; copied < copy && i < iov_cnt; i++) {
+		size_t len = MIN(iov[i].iov_len - offset, copy - copied);
 
 		memcpy((char *)iov[i].iov_base + offset, (char *)buf + copied,
 		       len);
 		copied += len;
+
+		if (copied < copy)
+			offset = 0;	/* More to copy in the next iteration */
+		else
+			offset = len;	/* Start of zero-filling, see below */
+	}
+
+	for (filled = 0; filled < fill && i < iov_cnt; i++) {
+		size_t len = MIN(iov[i].iov_len - offset, fill - filled);
+
+		memcpy((char *)iov[i].iov_base + offset,
+		       (char *)buf + copied + filled, len);
+		filled += len;
 		offset = 0;
 	}
 
-	return copied;
+	return copied + filled;
 }
 
 /**
@@ -96,27 +112,27 @@ size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
  * @iov_cnt:	Number of elements in iovec array
  * @offset:	Source offset altogether, counted in flattened iovec
  * @buf:	Destination buffer
- * @bytes:	Bytes to copy
+ * @copy:	Bytes to copy
  *
  * Return: number of bytes copied
  */
 size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
-		  size_t offset, void *buf, size_t bytes)
+		  size_t offset, void *buf, size_t copy)
 {
 	unsigned int i;
 	size_t copied;
 
-	if (__builtin_constant_p(bytes) && iov_cnt &&
-		offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
-		memcpy(buf, (char *)iov[0].iov_base + offset, bytes);
+	if (__builtin_constant_p(copy) && iov_cnt &&
+		offset <= iov[0].iov_len && copy <= iov[0].iov_len - offset) {
+		memcpy(buf, (char *)iov[0].iov_base + offset, copy);
 
-		return bytes;
+		return copy;
 	}
 
 	i = iov_skip_bytes(iov, iov_cnt, offset, &offset);
 
-	for (copied = 0; copied < bytes && i < iov_cnt; i++) {
-		size_t len = MIN(iov[i].iov_len - offset, bytes - copied);
+	for (copied = 0; copied < copy && i < iov_cnt; i++) {
+		size_t len = MIN(iov[i].iov_len - offset, copy - copied);
 
 		ASSERT(iov[i].iov_base);
 
diff --git a/iov.h b/iov.h
index ba1fda5..9b5d910 100644
--- a/iov.h
+++ b/iov.h
@@ -24,9 +24,9 @@
 size_t iov_skip_bytes(const struct iovec *iov, size_t n,
 		      size_t skip, size_t *offset);
 size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
-                    size_t offset, const void *buf, size_t bytes);
+                    size_t offset, const void *buf, size_t copy, size_t fill);
 size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
-                  size_t offset, void *buf, size_t bytes);
+                  size_t offset, void *buf, size_t copy);
 size_t iov_size(const struct iovec *iov, size_t iov_cnt);
 
 /*
diff --git a/vu_common.c b/vu_common.c
index 21cfb2a..d941b72 100644
--- a/vu_common.c
+++ b/vu_common.c
@@ -274,7 +274,7 @@ int vu_send_single(const struct ctx *c, const void *buf, size_t size)
 
 	/* copy data from the buffer to the iovec */
 	iov_from_buf(in_sg, elem_cnt, sizeof(struct virtio_net_hdr_mrg_rxbuf),
-		     buf, size);
+		     buf, size, vu_buf_size - size);
 
 	if (*c->pcap) {
 		pcap_iov(in_sg, elem_cnt,
-- 
2.43.0