Re: [Qemu-devel] [PATCH v2 1/5] virtio-net: use the backend cross-endian capabilities

[Laurent Vivier <lvivier@redhat.com>]

On 11/01/2016 17:12, 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 (i.e. drivers sets the CONFIG_OK bit).
> If the backend cannot support the requested endiannes, we have to fallback
> onto virtio_net_hdr_swap(): this is recorded in the needs_vnet_hdr_swap flag,
> to be used in the TX and RX paths.
> 
> Note that we reset the backend to the default behaviour (guest native
> endianness) when the device stops (i.e. device status had CONFIG_OK bit and
> driver unsets it). This is needed, with the linux tap backend at least,
> otherwise the guest may loose network connectivity if rebooted into a
> different endianness.
> 
> The current vhost-net code also tries to configure net backends. This will
> be no more needed and will be reverted in a subsequent patch.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
> v2:
> - dropped useless check in the 'else if' branch in virtio_net_vnet_status()
> - merged virtio_net_vhost_status() change from patch 2
> - use semicolon in "backend does no support..." error message
> - merged patch 3 (drop the virtio_needs_swap() helper)
> - provided some more details in changelog and comments
> ---
>  hw/net/virtio-net.c               |   49 +++++++++++++++++++++++++++++++++++--
>  include/hw/virtio/virtio-access.h |    9 -------
>  include/hw/virtio/virtio-net.h    |    1 +
>  3 files changed, 48 insertions(+), 11 deletions(-)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index a877614e3e7a..497fb7119a08 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -128,6 +128,13 @@ static void virtio_net_vhost_status(VirtIONet *n, uint8_t status)
>      if (!n->vhost_started) {
>          int r, i;
>  
> +        if (n->needs_vnet_hdr_swap) {
> +            error_report("backend does not support %s vnet headers; "
> +                         "falling back on userspace virtio",
> +                         virtio_is_big_endian(vdev) ? "BE" : "LE");
> +            return;
> +        }
> +
>          /* Any packets outstanding? Purge them to avoid touching rings
>           * when vhost is running.
>           */
> @@ -152,6 +159,40 @@ 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;
> +
> +        /* Before using the device, we tell the network backend about the
> +         * endianness to use when parsing vnet headers. If the backend can't
> +         * do it, we fallback onto fixing the headers in the core virtio-net
> +         * code.
> +         */
> +        if (virtio_is_big_endian(vdev)) {
> +            r = qemu_set_vnet_be(peer, true);
> +        } else {
> +            r = qemu_set_vnet_le(peer, true);
> +        }

If endianess of the guest and the virtio device is the same, but r is <
0 (-ENOSYS or -EINVAL) you will badly swap header (and disable vhost).

I think you need something like this to fall back to the old method:

        if (r < 0) {
#ifdef HOST_WORDS_BIGENDIAN
            r = virtio_access_is_big_endian(vdev) ? false : true;
#else
            r = virtio_access_is_big_endian(vdev) ? true : false;
#endif
        }


But...

> +        n->needs_vnet_hdr_swap = !!r;
> +    } else if (virtio_net_started(n, vdev->status)) {
> +        /* After using the device, we need to reset the network backend to
> +         * the default (guest native endianness), otherwise the guest may
> +         * loose network connectivity if it is rebooted into a different
> +         * endianness.
> +         */
> +        if (virtio_is_big_endian(vdev)) {
> +            qemu_set_vnet_be(peer, false);
> +        } else {
> +            qemu_set_vnet_le(peer, false);
> +        }
> +    }
> +}
> +
>  static void virtio_net_set_status(struct VirtIODevice *vdev, uint8_t status)
>  {
>      VirtIONet *n = VIRTIO_NET(vdev);
> @@ -159,6 +200,7 @@ 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 +999,10 @@ 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);
> +        }

... this will change the behavior here, as before it was not
conditional. Why ?

>          iov_from_buf(iov, iov_cnt, 0, buf, sizeof(struct virtio_net_hdr));
>      } else {
>          struct virtio_net_hdr hdr = {
> @@ -1167,7 +1212,7 @@ 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-access.h b/include/hw/virtio/virtio-access.h
> index 8aec843c8ff3..a01fff2e51d7 100644
> --- a/include/hw/virtio/virtio-access.h
> +++ b/include/hw/virtio/virtio-access.h
> @@ -143,15 +143,6 @@ static inline uint64_t virtio_ldq_p(VirtIODevice *vdev, const void *ptr)
>      }
>  }
>  
> -static inline bool virtio_needs_swap(VirtIODevice *vdev)
> -{
> -#ifdef HOST_WORDS_BIGENDIAN
> -    return virtio_access_is_big_endian(vdev) ? false : true;
> -#else
> -    return virtio_access_is_big_endian(vdev) ? true : false;
> -#endif
> -}
> -
>  static inline uint16_t virtio_tswap16(VirtIODevice *vdev, uint16_t s)
>  {
>  #ifdef HOST_WORDS_BIGENDIAN
> 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,
> 
>