[PATCH v4 05/14] vringh: implement vringh_kiov_advance()

[Stefano Garzarella]

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().

Acked-by: Jason Wang <jasow...@redhat.com>
Signed-off-by: Stefano Garzarella <sgarz...@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 2a88e087afd8..4af8fa259d65 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;
 }
-- 
2.30.2