Re: [PATCH RFC 11/12] vringh: allow vringh_iov_xfer() to skip bytes when ptr is NULL

[Jason Wang]

On 2020/11/13 下午9:47, Stefano Garzarella wrote:
In some cases, it may be useful to provide a way to skip a number
of bytes in a vringh_iov.

In order to keep vringh_iov consistent, let's reuse vringh_iov_xfer()
logic and skip bytes when the ptr is NULL.

Signed-off-by: Stefano Garzarella <sgarz...@redhat.com>
---

I'm not sure if this is the best option, maybe we can add a new
function vringh_iov_skip().

Suggestions?


I might be worth to check whether we can convert vringh_iov to use iov 
iterator then we can use iov_iterator_advance() here.

Thanks


---
  drivers/vhost/vringh.c | 16 +++++++++++-----
  1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
index 8bd8b403f087..ed3290946ad7 100644
--- a/drivers/vhost/vringh.c
+++ b/drivers/vhost/vringh.c
@@ -75,7 +75,9 @@ static inline int __vringh_get_head(const struct vringh *vrh,
        return head;
  }
  
-/* Copy some bytes to/from the iovec.  Returns num copied. */
+/* Copy some bytes to/from the iovec.  Returns num copied.
+ * If ptr is NULL, skips at most len bytes.
+ */
  static inline ssize_t vringh_iov_xfer(struct vringh *vrh,
                                      struct vringh_kiov *iov,
                                      void *ptr, size_t len,
@@ -89,12 +91,16 @@ static inline ssize_t vringh_iov_xfer(struct vringh *vrh,
                size_t partlen;
  
  		partlen = min(iov->iov[iov->i].iov_len, len);
-               err = xfer(vrh, iov->iov[iov->i].iov_base, ptr, partlen);
-               if (err)
-                       return err;
+
+               if (ptr) {
+                       err = xfer(vrh, iov->iov[iov->i].iov_base, ptr, 
partlen);
+                       if (err)
+                               return err;
+                       ptr += partlen;
+               }
+
                done += partlen;
                len -= partlen;
-               ptr += partlen;
                iov->consumed += partlen;
                iov->iov[iov->i].iov_len -= partlen;
                iov->iov[iov->i].iov_base += partlen;