iov: Add helper to find skip over first n bytes of an io vector

Several of the IOV functions in iov.c, and also tap_send_frames_passt()
needs to determine which buffer element a byte offset into an IO vector
lies in.  Split this out into a helper function iov_skip_bytes().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

iov.c

@@ -25,6 +25,36 @@
 #include "util.h"
 #include "iov.h"
 
+
+/* iov_skip_bytes() - Skip the first n bytes into an IO vector
+ * @iov:	IO vector
+ * @n:		Number of entries in @iov
+ * @vec_offset: Total byte offset into the IO vector
+ * @buf_offset:	Offset into a single buffer of the IO vector
+ *
+ * Return: index I of individual struct iovec which contains the byte at
+ *         @vec_offset bytes into the vector (as though all its buffers were
+ *         contiguous).  If @buf_offset is non-NULL, update it to the offset of
+ *         that byte within @iov[I] (guaranteed to be less than @iov[I].iov_len)
+ *	   If the whole vector has <= @vec_offset bytes, return @n.
+ */
+size_t iov_skip_bytes(const struct iovec *iov, size_t n,
+		      size_t vec_offset, size_t *buf_offset)
+{
+	size_t offset = vec_offset, i;
+
+	for (i = 0; i < n; i++) {
+		if (offset < iov[i].iov_len)
+			break;
+		offset -= iov[i].iov_len;
+	}
+
+	if (buf_offset)
+		*buf_offset = offset;
+
+	return i;
+}
+
 /**
  * iov_from_buf - Copy data from a buffer to an I/O vector (struct iovec)
  *                efficiently.
@@ -51,9 +81,7 @@ size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
 		return bytes;
 	}
 
-	/* skipping offset bytes in the iovec */
+	i = iov_skip_bytes(iov, iov_cnt, offset, &offset);
-	for (i = 0; i < iov_cnt && offset >= iov[i].iov_len; i++)
-		offset -= iov[i].iov_len;
 
 	/* copying data */
 	for (copied = 0; copied < bytes && i < iov_cnt; i++) {
@@ -94,9 +122,7 @@ size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
 		return bytes;
 	}
 
-	/* skipping offset bytes in the iovec */
+	i = iov_skip_bytes(iov, iov_cnt, offset, &offset);
-	for (i = 0; i < iov_cnt && offset >= iov[i].iov_len; i++)
-		offset -= iov[i].iov_len;
 
 	/* copying data */
 	for (copied = 0; copied < bytes && i < iov_cnt; i++) {
@@ -155,9 +181,7 @@ unsigned iov_copy(struct iovec *dst_iov, size_t dst_iov_cnt,
 {
 	unsigned int i, j;
 
-	/* skipping offset bytes in the iovec */
+	i = iov_skip_bytes(iov, iov_cnt, offset, &offset);
-	for (i = 0; i < iov_cnt && offset >= iov[i].iov_len; i++)
-		offset -= iov[i].iov_len;
 
 	/* copying data */
 	for (j = 0; i < iov_cnt && j < dst_iov_cnt && bytes; i++) {

iov.h

@@ -18,6 +18,8 @@
 #include <unistd.h>
 #include <string.h>
 
+size_t iov_skip_bytes(const struct iovec *iov, size_t n,
+		      size_t vec_offset, size_t *buf_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 iov_to_buf(const struct iovec *iov, size_t iov_cnt,

tap.c

@@ -45,6 +45,7 @@
 
 #include "checksum.h"
 #include "util.h"
+#include "iov.h"
 #include "passt.h"
 #include "arp.h"
 #include "dhcp.h"
@@ -389,6 +390,7 @@ static size_t tap_send_frames_passt(const struct ctx *c,
 		.msg_iov = (void *)iov,
 		.msg_iovlen = n,
 	};
+	size_t buf_offset;
 	unsigned int i;
 	ssize_t sent;
 
@@ -397,15 +399,11 @@ static size_t tap_send_frames_passt(const struct ctx *c,
 		return 0;
 
 	/* Check for any partial frames due to short send */
-	for (i = 0; i < n; i++) {
+	i = iov_skip_bytes(iov, n, sent, &buf_offset);
-		if ((size_t)sent < iov[i].iov_len)
-			break;
-		sent -= iov[i].iov_len;
-	}
 
-	if (i < n && sent) {
+	if (i < n && buf_offset) {
 		/* A partial frame was sent */
-		tap_send_remainder(c, &iov[i], sent);
+		tap_send_remainder(c, &iov[i], buf_offset);
 		i++;
 	}