vhost: don't bother copying iovecs in handle_rx(), kill memcpy_toiovecend()

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: kvm@vger.kernel.org
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/vhost/net.c

@@ -84,10 +84,6 @@ struct vhost_net_ubuf_ref {
 
 struct vhost_net_virtqueue {
 	struct vhost_virtqueue vq;
-	/* hdr is used to store the virtio header.
-	 * Since each iovec has >= 1 byte length, we never need more than
-	 * header length entries to store the header. */
-	struct iovec hdr[sizeof(struct virtio_net_hdr_mrg_rxbuf)];
 	size_t vhost_hlen;
 	size_t sock_hlen;
 	/* vhost zerocopy support fields below: */
@@ -235,44 +231,6 @@ static bool vhost_sock_zcopy(struct socket *sock)
 		sock_flag(sock->sk, SOCK_ZEROCOPY);
 }
 
-/* Pop first len bytes from iovec. Return number of segments used. */
-static int move_iovec_hdr(struct iovec *from, struct iovec *to,
-			  size_t len, int iov_count)
-{
-	int seg = 0;
-	size_t size;
-
-	while (len && seg < iov_count) {
-		size = min(from->iov_len, len);
-		to->iov_base = from->iov_base;
-		to->iov_len = size;
-		from->iov_len -= size;
-		from->iov_base += size;
-		len -= size;
-		++from;
-		++to;
-		++seg;
-	}
-	return seg;
-}
-/* Copy iovec entries for len bytes from iovec. */
-static void copy_iovec_hdr(const struct iovec *from, struct iovec *to,
-			   size_t len, int iovcount)
-{
-	int seg = 0;
-	size_t size;
-
-	while (len && seg < iovcount) {
-		size = min(from->iov_len, len);
-		to->iov_base = from->iov_base;
-		to->iov_len = size;
-		len -= size;
-		++from;
-		++to;
-		++seg;
-	}
-}
-
 /* In case of DMA done not in order in lower device driver for some reason.
  * upend_idx is used to track end of used idx, done_idx is used to track head
  * of used idx. Once lower device DMA done contiguously, we will signal KVM
@@ -570,9 +528,9 @@ static void handle_rx(struct vhost_net *net)
 		.msg_controllen = 0,
 		.msg_flags = MSG_DONTWAIT,
 	};
-	struct virtio_net_hdr_mrg_rxbuf hdr = {
-		.hdr.flags = 0,
-		.hdr.gso_type = VIRTIO_NET_HDR_GSO_NONE
+	struct virtio_net_hdr hdr = {
+		.flags = 0,
+		.gso_type = VIRTIO_NET_HDR_GSO_NONE
 	};
 	size_t total_len = 0;
 	int err, mergeable;
@@ -580,6 +538,7 @@ static void handle_rx(struct vhost_net *net)
 	size_t vhost_hlen, sock_hlen;
 	size_t vhost_len, sock_len;
 	struct socket *sock;
+	struct iov_iter fixup;
 
 	mutex_lock(&vq->mutex);
 	sock = vq->private_data;
@@ -624,14 +583,19 @@ static void handle_rx(struct vhost_net *net)
 			break;
 		}
 		/* We don't need to be notified again. */
-		if (unlikely((vhost_hlen)))
-			/* Skip header. TODO: support TSO. */
-			move_iovec_hdr(vq->iov, nvq->hdr, vhost_hlen, in);
-		else
-			/* Copy the header for use in VIRTIO_NET_F_MRG_RXBUF:
-			 * needed because recvmsg can modify msg_iov. */
-			copy_iovec_hdr(vq->iov, nvq->hdr, sock_hlen, in);
-		iov_iter_init(&msg.msg_iter, READ, vq->iov, in, sock_len);
+		iov_iter_init(&msg.msg_iter, READ, vq->iov, in, vhost_len);
+		fixup = msg.msg_iter;
+		if (unlikely((vhost_hlen))) {
+			/* We will supply the header ourselves
+			 * TODO: support TSO.
+			 */
+			iov_iter_advance(&msg.msg_iter, vhost_hlen);
+		} else {
+			/* It'll come from socket; we'll need to patch
+			 * ->num_buffers over if VIRTIO_NET_F_MRG_RXBUF
+			 */
+			iov_iter_advance(&fixup, sizeof(hdr));
+		}
 		err = sock->ops->recvmsg(NULL, sock, &msg,
 					 sock_len, MSG_DONTWAIT | MSG_TRUNC);
 		/* Userspace might have consumed the packet meanwhile:
@@ -643,18 +607,18 @@ static void handle_rx(struct vhost_net *net)
 			vhost_discard_vq_desc(vq, headcount);
 			continue;
 		}
+		/* Supply virtio_net_hdr if VHOST_NET_F_VIRTIO_NET_HDR */
 		if (unlikely(vhost_hlen) &&
-		    memcpy_toiovecend(nvq->hdr, (unsigned char *)&hdr, 0,
-				      vhost_hlen)) {
+		    copy_to_iter(&hdr, sizeof(hdr), &fixup) != sizeof(hdr)) {
 			vq_err(vq, "Unable to write vnet_hdr at addr %p\n",
 			       vq->iov->iov_base);
 			break;
 		}
-		/* TODO: Should check and handle checksum. */
+		/* Supply (or replace) ->num_buffers if VIRTIO_NET_F_MRG_RXBUF
+		 * TODO: Should check and handle checksum.
+		 */
 		if (likely(mergeable) &&
-		    memcpy_toiovecend(nvq->hdr, (unsigned char *)&headcount,
-				      offsetof(typeof(hdr), num_buffers),
-				      sizeof hdr.num_buffers)) {
+		    copy_to_iter(&headcount, 2, &fixup) != 2) {
 			vq_err(vq, "Failed num_buffers write");
 			vhost_discard_vq_desc(vq, headcount);
 			break;

include/linux/uio.h

@@ -137,7 +137,4 @@ size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum, struct io
 
 int memcpy_fromiovecend(unsigned char *kdata, const struct iovec *iov,
 			int offset, int len);
-int memcpy_toiovecend(const struct iovec *v, unsigned char *kdata,
-		      int offset, int len);
-
 #endif

lib/iovec.c

@@ -2,32 +2,6 @@
 #include <linux/export.h>
 #include <linux/uio.h>
 
-/*
- *	Copy kernel to iovec. Returns -EFAULT on error.
- */
-
-int memcpy_toiovecend(const struct iovec *iov, unsigned char *kdata,
-		      int offset, int len)
-{
-	int copy;
-	for (; len > 0; ++iov) {
-		/* Skip over the finished iovecs */
-		if (unlikely(offset >= iov->iov_len)) {
-			offset -= iov->iov_len;
-			continue;
-		}
-		copy = min_t(unsigned int, iov->iov_len - offset, len);
-		if (copy_to_user(iov->iov_base + offset, kdata, copy))
-			return -EFAULT;
-		offset = 0;
-		kdata += copy;
-		len -= copy;
-	}
-
-	return 0;
-}
-EXPORT_SYMBOL(memcpy_toiovecend);
-
 /*
  *	Copy iovec to kernel. Returns -EFAULT on error.
  */