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From:   Stefano Garzarella <sgarzare@redhat.com>
To:     virtualization@lists.linux-foundation.org
Cc:     Stefan Hajnoczi <stefanha@redhat.com>,
        Laurent Vivier <lvivier@redhat.com>,
        linux-kernel@vger.kernel.org, Eli Cohen <elic@nvidia.com>,
        Jason Wang <jasowang@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Max Gurtovoy <mgurtovoy@nvidia.com>
Subject: [PATCH RFC 11/12] vringh: allow vringh_iov_xfer() to skip bytes when ptr is NULL
Date:   Fri, 13 Nov 2020 14:47:11 +0100
Message-Id: <20201113134712.69744-12-sgarzare@redhat.com>
In-Reply-To: <20201113134712.69744-1-sgarzare@redhat.com>
References: <20201113134712.69744-1-sgarzare@redhat.com>
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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 <sgarzare@redhat.com>
---

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

Suggestions?
---
 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;
-- 
2.26.2