[PATCH RFC v2 04/10] vringh: implement vringh_kiov_advance()

[Stefano Garzarella <sgarzare@redhat.com>]

In some cases, it may be useful to provide a way to skip a number
of bytes in a vringh_kiov.

Let's implement vringh_kiov_advance() for this purpose, reusing the
code from vringh_iov_xfer().
We replace that code calling the new vringh_kiov_advance().

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
 include/linux/vringh.h |  2 ++
 drivers/vhost/vringh.c | 41 +++++++++++++++++++++++++++++------------
 2 files changed, 31 insertions(+), 12 deletions(-)

diff --git a/include/linux/vringh.h b/include/linux/vringh.h
index 9c077863c8f6..755211ebd195 100644
--- a/include/linux/vringh.h
+++ b/include/linux/vringh.h
@@ -199,6 +199,8 @@ static inline void vringh_kiov_cleanup(struct vringh_kiov *kiov)
 	kiov->iov = NULL;
 }
 
+void vringh_kiov_advance(struct vringh_kiov *kiov, size_t len);
+
 int vringh_getdesc_kern(struct vringh *vrh,
 			struct vringh_kiov *riov,
 			struct vringh_kiov *wiov,
diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
index bee63d68201a..4d800e4f31ca 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -75,6 +75,34 @@ static inline int __vringh_get_head(const struct vringh *vrh,
 	return head;
 }
 
+/**
+ * vringh_kiov_advance - skip bytes from vring_kiov
+ * @iov: an iov passed to vringh_getdesc_*() (updated as we consume)
+ * @len: the maximum length to advance
+ */
+void vringh_kiov_advance(struct vringh_kiov *iov, size_t len)
+{
+	while (len && iov->i < iov->used) {
+		size_t partlen = min(iov->iov[iov->i].iov_len, len);
+
+		iov->consumed += partlen;
+		iov->iov[iov->i].iov_len -= partlen;
+		iov->iov[iov->i].iov_base += partlen;
+
+		if (!iov->iov[iov->i].iov_len) {
+			/* Fix up old iov element then increment. */
+			iov->iov[iov->i].iov_len = iov->consumed;
+			iov->iov[iov->i].iov_base -= iov->consumed;
+
+			iov->consumed = 0;
+			iov->i++;
+		}
+
+		len -= partlen;
+	}
+}
+EXPORT_SYMBOL(vringh_kiov_advance);
+
 /* Copy some bytes to/from the iovec.  Returns num copied. */
 static inline ssize_t vringh_iov_xfer(struct vringh *vrh,
 				      struct vringh_kiov *iov,
@@ -95,19 +123,8 @@ static inline ssize_t vringh_iov_xfer(struct vringh *vrh,
 		done += partlen;
 		len -= partlen;
 		ptr += partlen;
-		iov->consumed += partlen;
-		iov->iov[iov->i].iov_len -= partlen;
-		iov->iov[iov->i].iov_base += partlen;
 
-		if (!iov->iov[iov->i].iov_len) {
-			/* Fix up old iov element then increment. */
-			iov->iov[iov->i].iov_len = iov->consumed;
-			iov->iov[iov->i].iov_base -= iov->consumed;
-
-			
-			iov->consumed = 0;
-			iov->i++;
-		}
+		vringh_kiov_advance(iov, partlen);
 	}
 	return done;
 }
-- 
2.29.2