Re: [Qemu-devel] [PATCH 1/6] virtio-net: use the backend cross-endian capabilities

[Laurent Vivier]

On 07/01/2016 12:32, Greg Kurz wrote:
> When running a fully emulated device in cross-endian conditions, including
> a virtio 1.0 device offered to a big endian guest, we need to fix the vnet
> headers. This is currently handled by the virtio_net_hdr_swap() function
> in the core virtio-net code but it should actually be handled by the net
> backend.
> 
> With this patch, virtio-net now tries to configure the backend to do the
> endian fixing when the device starts. If the backend cannot support the
> requested endiannes, we have to fall back on virtio_net_hdr_swap(): this
> is recorded in the needs_vnet_hdr_swap flag, to be used in the TX and RX
> paths.
> 
> The current vhost-net code also tries to configure net backends. This will
> be no more needed and will be addressed in a subsequent patch.
> 
> Signed-off-by: Greg Kurz <address@hidden>
> ---
>  hw/net/virtio-net.c            |   33 +++++++++++++++++++++++++++++++--
>  include/hw/virtio/virtio-net.h |    1 +
>  2 files changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index a877614e3e7a..d4cc94ea5e55 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -152,6 +152,31 @@ static void virtio_net_vhost_status(VirtIONet *n, 
> uint8_t status)
>      }
>  }
>  
> +static void virtio_net_vnet_status(VirtIONet *n, uint8_t status)
> +{
> +    VirtIODevice *vdev = VIRTIO_DEVICE(n);
> +    NetClientState *peer = qemu_get_queue(n->nic)->peer;
> +
> +    if (virtio_net_started(n, status)) {
> +        int r;
> +
> +        if (virtio_is_big_endian(vdev)) {
> +            r = qemu_set_vnet_be(peer, true);
> +        } else {
> +            r = qemu_set_vnet_le(peer, true);
> +        }
> +
> +        n->needs_vnet_hdr_swap = !!r;
> +    } else if (virtio_net_started(n, vdev->status) &&
> +               !virtio_net_started(n, status)) {
> +        if (virtio_is_big_endian(vdev)) {
> +            qemu_set_vnet_be(peer, false);
> +        } else {
> +            qemu_set_vnet_le(peer, false);
> +        }
> +    }
> +}

Could you explain why check 'virtio_net_started(n, status)' and then
'virtio_net_started(n, vdev->status)' ?

Why qemu_set_vnet_[bl]e() use "true" in the first case and "false" in
the second case ?

Why don't you store the result (r) in the second case ?

>  static void virtio_net_set_status(struct VirtIODevice *vdev, uint8_t status)
>  {
>      VirtIONet *n = VIRTIO_NET(vdev);
> @@ -159,6 +184,7 @@ static void virtio_net_set_status(struct VirtIODevice 
> *vdev, uint8_t status)
>      int i;
>      uint8_t queue_status;
>  
> +    virtio_net_vnet_status(n, status);
>      virtio_net_vhost_status(n, status);
>  
>      for (i = 0; i < n->max_queues; i++) {
> @@ -957,7 +983,10 @@ static void receive_header(VirtIONet *n, const struct 
> iovec *iov, int iov_cnt,
>          void *wbuf = (void *)buf;
>          work_around_broken_dhclient(wbuf, wbuf + n->host_hdr_len,
>                                      size - n->host_hdr_len);
> -        virtio_net_hdr_swap(VIRTIO_DEVICE(n), wbuf);
> +
> +        if (n->needs_vnet_hdr_swap) {
> +            virtio_net_hdr_swap(VIRTIO_DEVICE(n), wbuf);
> +        }
>          iov_from_buf(iov, iov_cnt, 0, buf, sizeof(struct virtio_net_hdr));
>      } else {
>          struct virtio_net_hdr hdr = {
> @@ -1167,7 +1196,7 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
>                  error_report("virtio-net header incorrect");
>                  exit(1);
>              }
> -            if (virtio_needs_swap(vdev)) {
> +            if (n->needs_vnet_hdr_swap) {
>                  virtio_net_hdr_swap(vdev, (void *) &mhdr);
>                  sg2[0].iov_base = &mhdr;
>                  sg2[0].iov_len = n->guest_hdr_len;
> diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
> index f3cc25feca2b..27bc868fbc7d 100644
> --- a/include/hw/virtio/virtio-net.h
> +++ b/include/hw/virtio/virtio-net.h
> @@ -94,6 +94,7 @@ typedef struct VirtIONet {
>      uint64_t curr_guest_offloads;
>      QEMUTimer *announce_timer;
>      int announce_counter;
> +    bool needs_vnet_hdr_swap;
>  } VirtIONet;
>  
>  void virtio_net_set_netclient_name(VirtIONet *n, const char *name,
> 
>