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Date: Tue, 3 Feb 2015 12:04:12 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Al Viro <viro@...IV.linux.org.uk>
Cc: David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
kvm@...r.kernel.org
Subject: Re: [PATCH v2 17/18] vhost: don't bother copying iovecs in
handle_rx(), kill memcpy_toiovecend()
On Mon, Feb 02, 2015 at 07:59:36AM +0000, Al Viro wrote:
> From: Al Viro <viro@...iv.linux.org.uk>
>
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Cc: kvm@...r.kernel.org
> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> ---
> drivers/vhost/net.c | 79 ++++++++++++++---------------------------------------
> include/linux/uio.h | 3 --
> lib/iovec.c | 26 ------------------
> 3 files changed, 20 insertions(+), 88 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index d86cc9b..73c0ebf 100644
> --- a/drivers/vhost/net.c
> +++ b/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 +583,17 @@ 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:
Hmm having second thoughts here.
Will this modify the iov in vq->iov?
If yes, how will recvmsg fill it?
> @@ -643,18 +605,17 @@ 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;
Here, if recvmsg modified the iov, how does copy_to_iter
see the header?
> diff --git a/include/linux/uio.h b/include/linux/uio.h
> index af3439f..02bd8a9 100644
> --- a/include/linux/uio.h
> +++ b/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
> diff --git a/lib/iovec.c b/lib/iovec.c
> index 4a90875..d8f17a9 100644
> --- a/lib/iovec.c
> +++ b/lib/iovec.c
> @@ -3,32 +3,6 @@
> #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.
> */
>
> --
> 2.1.4
--
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