Re: [Qemu-devel] [PATCH 04/14] virtio-net: avoid sg copy

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Sep 24, 2012 at 07:37:40PM -0500, Anthony Liguori wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > Avoid tweaking iovec during receive. This removes
> > the need to copy the vector.
> > Note: we already have an evil cast in work_around_broken_dhclient
> > and unfortunately this adds another one.
> > const on buf is ignored by this function anyway so arguably
> > this is not making things much worse.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >  hw/virtio-net.c | 45 +++++++++++++++++++++------------------------
> >  iov.h           |  8 ++++----
> >  2 files changed, 25 insertions(+), 28 deletions(-)
> >
> > diff --git a/hw/virtio-net.c b/hw/virtio-net.c
> > index 0c5081e..6e53858 100644
> > --- a/hw/virtio-net.c
> > +++ b/hw/virtio-net.c
> > @@ -503,7 +503,7 @@ static int virtio_net_has_buffers(VirtIONet *n, int bufsize)
> >   * we should provide a mechanism to disable it to avoid polluting the host
> >   * cache.
> >   */
> > -static void work_around_broken_dhclient(struct virtio_net_hdr *hdr,
> > +static void work_around_broken_dhclient(const struct virtio_net_hdr *hdr,
> >                                          const uint8_t *buf, size_t size)
> >  {
> >      if ((hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && /* missing csum */
> > @@ -513,31 +513,27 @@ static void work_around_broken_dhclient(struct virtio_net_hdr *hdr,
> >          (buf[34] == 0 && buf[35] == 67)) { /* udp.srcport == bootps */
> >          /* FIXME this cast is evil */
> >          net_checksum_calculate((uint8_t *)buf, size);
> > -        hdr->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM;
> > +        ((struct virtio_net_hdr *)hdr)->flags &=
> > ~VIRTIO_NET_HDR_F_NEEDS_CSUM;
> >      }
> >  }
> >  
> > -static int receive_header(VirtIONet *n, struct iovec *iov, int iovcnt,
> > -                          const void *buf, size_t size, size_t hdr_len)
> > +static int receive_header(VirtIONet *n, const struct iovec *iov, int iov_cnt,
> > +                          const void *buf, size_t size)
> >  {
> > -    struct virtio_net_hdr *hdr = (struct virtio_net_hdr *)iov[0].iov_base;
> >      int offset = 0;
> >  
> > -    hdr->flags = 0;
> > -    hdr->gso_type = VIRTIO_NET_HDR_GSO_NONE;
> > -
> >      if (n->has_vnet_hdr) {
> > -        memcpy(hdr, buf, sizeof(*hdr));
> > -        offset = sizeof(*hdr);
> > -        work_around_broken_dhclient(hdr, buf + offset, size - offset);
> > +        work_around_broken_dhclient(buf, buf + offset, size -
> > offset);
> 
> I think it would be less ugly to cast here.

I was thinking about this but could not decide either way.
Will do.

> > +        offset = sizeof(struct virtio_net_hdr);
> > +        iov_from_buf(iov, iov_cnt, 0, buf, offset);
> > +    } else {
> > +        struct virtio_net_hdr hdr = {
> > +            .flags = 0,
> > +            .gso_type = VIRTIO_NET_HDR_GSO_NONE
> > +        };
> > +        iov_from_buf(iov, iov_cnt, 0, &hdr, sizeof hdr);
> >      }
> >  
> > -    /* We only ever receive a struct virtio_net_hdr from the tapfd,
> > -     * but we may be passing along a larger header to the guest.
> > -     */
> > -    iov[0].iov_base += hdr_len;
> > -    iov[0].iov_len  -= hdr_len;
> > -
> >      return offset;
> >  }
> >  
> > @@ -598,7 +594,8 @@ static ssize_t virtio_net_receive(NetClientState *nc, const uint8_t *buf, size_t
> >  {
> >      VirtIONet *n = DO_UPCAST(NICState, nc, nc)->opaque;
> >      struct virtio_net_hdr_mrg_rxbuf *mhdr = NULL;
> > -    size_t offset, i;
> > +    const struct iovec *sg = elem.in_sg;
> > +    size_t offset, i, guest_offset;
> >  
> >      if (!virtio_net_can_receive(&n->nic->nc))
> >          return -1;
> > @@ -615,7 +612,7 @@ static ssize_t virtio_net_receive(NetClientState *nc, const uint8_t *buf, size_t
> >      while (offset < size) {
> >          VirtQueueElement elem;
> >          int len, total;
> > -        struct iovec sg[VIRTQUEUE_MAX_SIZE];
> > +        const struct iovec *sg = elem.in_sg;
> >  
> >          total = 0;
> >  
> > @@ -640,20 +637,20 @@ static ssize_t virtio_net_receive(NetClientState *nc, const uint8_t *buf, size_t
> >              exit(1);
> >          }
> >  
> > -        memcpy(&sg, &elem.in_sg[0], sizeof(sg[0]) * elem.in_num);
> > -
> >          if (i == 0) {
> >              if (n->mergeable_rx_bufs)
> >                  mhdr = (struct virtio_net_hdr_mrg_rxbuf *)sg[0].iov_base;
> >  
> >              offset += receive_header(n, sg, elem.in_num,
> > -                                     buf + offset, size - offset,
> > -                                     n->guest_hdr_len);
> > +                                     buf + offset, size - offset);
> >              total += n->guest_hdr_len;
> > +            guest_offset = n->guest_hdr_len;
> > +        } else {
> > +            guest_offset = 0;
> >          }
> >  
> >          /* copy in packet.  ugh */
> > -        len = iov_from_buf(sg, elem.in_num, 0,
> > +        len = iov_from_buf(sg, elem.in_num, guest_offset,
> >                             buf + offset, size - offset);
> >          total += len;
> >          offset += len;
> > diff --git a/iov.h b/iov.h
> > index f0daefa..6c9fdf1 100644
> > --- a/iov.h
> > +++ b/iov.h
> > @@ -90,8 +90,8 @@ void iov_hexdump(const struct iovec *iov, const unsigned int iov_cnt,
> >  /*
> >   * Partial copy of vector from iov to dst_iov (data is not copied).
> >   * dst_iov overlaps iov at a specified offset.
> > - * size of dst_iov is at most bytes. Actual size is returned.
> > + * size of dst_iov is at most bytes. dst vector count is returned.
> >   */
> > -size_t iov_cpy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
> > -               const struct iovec *iov, unsigned int iov_cnt,
> > -               size_t offset, size_t bytes);
> > +unsigned iov_cpy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
> > +                 const struct iovec *iov, unsigned int iov_cnt,
> > +                 size_t offset, size_t bytes);
> 
> I think this belongs in an earlier patch.

Good catch thanks.

> Regards,
> 
> Anthony Liguori
> 
> > -- 
> > MST