Re: [Qemu-devel] [PATCH v2] virtio_net: Bypass backends for MTU feature negotiation

[Vlad Yasevich]

On 05/23/2017 08:31 AM, Maxime Coquelin wrote:
> This patch adds a new internal "x-mtu-bypass-backend" property
> to bypass backends for MTU feature negotiation.
> 
> When this property is set, the MTU feature is negotiated as soon
> as supported by the guest and a MTU value is set via the host_mtu
> parameter. In case the backend advertises the feature (e.g. DPDK's
> vhost-user backend), the feature negotiation is propagated down to
> the backend.
> 
> When this property is not set, the backend has to support the MTU
> feature for its negotiation to succeed.
> 
> For compatibility purpose, this property is disabled for machine
> types v2.9 and older.
> 
> Cc: Aaron Conole <address@hidden>
> Suggested-by: Michael S. Tsirkin <address@hidden>
> Signed-off-by: Maxime Coquelin <address@hidden>

Reviewed-by: Vlad Yasevich <address@hidden>

-vlad

> ---
> 
> Tests performed:
> - Vhost-net kernel backendi, host_mtu=2000:
>  * default machine type: guest MTU = 2000
>  * pc-q35-2.9 machine type: Guest MTU = 1500
> 
> - Vhost-net user backend, host_mtu=2000:
>  * DPDK v17.05 (MTU feature supported on backend side)
>   - default machine type: guest MTU = 2000
>   - pc-q35-2.9 machine type: guest MTU = 2000)
>  * DPDK v16.11 (MTU feature not supported on backend side)
>   - default machine type: guest MTU = 2000
>   - pc-q35-2.9 machine type: guest MTU = 1500)
> 
>  hw/net/virtio-net.c            | 17 ++++++++++++++++-
>  include/hw/compat.h            |  4 ++++
>  include/hw/virtio/virtio-net.h |  1 +
>  include/hw/virtio/virtio.h     |  1 +
>  4 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 7d091c9..39c336e 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -589,7 +589,15 @@ static uint64_t virtio_net_get_features(VirtIODevice 
> *vdev, uint64_t features,
>      if (!get_vhost_net(nc->peer)) {
>          return features;
>      }
> -    return vhost_net_get_features(get_vhost_net(nc->peer), features);
> +    features = vhost_net_get_features(get_vhost_net(nc->peer), features);
> +    vdev->backend_features = features;
> +
> +    if (n->mtu_bypass_backend &&
> +            (n->host_features & 1ULL << VIRTIO_NET_F_MTU)) {
> +        features |= (1ULL << VIRTIO_NET_F_MTU);
> +    }
> +
> +    return features;
>  }
>  
>  static uint64_t virtio_net_bad_features(VirtIODevice *vdev)
> @@ -640,6 +648,11 @@ static void virtio_net_set_features(VirtIODevice *vdev, 
> uint64_t features)
>      VirtIONet *n = VIRTIO_NET(vdev);
>      int i;
>  
> +    if (n->mtu_bypass_backend &&
> +            !virtio_has_feature(vdev->backend_features, VIRTIO_NET_F_MTU)) {
> +        features &= ~(1ULL << VIRTIO_NET_F_MTU);
> +    }
> +
>      virtio_net_set_multiqueue(n,
>                                virtio_has_feature(features, VIRTIO_NET_F_MQ));
>  
> @@ -2090,6 +2103,8 @@ static Property virtio_net_properties[] = {
>      DEFINE_PROP_UINT16("rx_queue_size", VirtIONet, net_conf.rx_queue_size,
>                         VIRTIO_NET_RX_QUEUE_DEFAULT_SIZE),
>      DEFINE_PROP_UINT16("host_mtu", VirtIONet, net_conf.mtu, 0),
> +    DEFINE_PROP_BOOL("x-mtu-bypass-backend", VirtIONet, mtu_bypass_backend,
> +                     true),
>      DEFINE_PROP_END_OF_LIST(),
>  };
>  
> diff --git a/include/hw/compat.h b/include/hw/compat.h
> index 55b1765..181f450 100644
> --- a/include/hw/compat.h
> +++ b/include/hw/compat.h
> @@ -6,6 +6,10 @@
>          .driver   = "pci-bridge",\
>          .property = "shpc",\
>          .value    = "off",\
> +    },{\
> +        .driver   = "virtio-net-device",\
> +        .property = "x-mtu-bypass-backend",\
> +        .value    = "off",\
>      },
>  
>  #define HW_COMPAT_2_8 \
> diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
> index 1eec9a2..602b486 100644
> --- a/include/hw/virtio/virtio-net.h
> +++ b/include/hw/virtio/virtio-net.h
> @@ -97,6 +97,7 @@ typedef struct VirtIONet {
>      QEMUTimer *announce_timer;
>      int announce_counter;
>      bool needs_vnet_hdr_swap;
> +    bool mtu_bypass_backend;
>  } VirtIONet;
>  
>  void virtio_net_set_netclient_name(VirtIONet *n, const char *name,
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index 7b6edba..80c45c3 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -79,6 +79,7 @@ struct VirtIODevice
>      uint16_t queue_sel;
>      uint64_t guest_features;
>      uint64_t host_features;
> +    uint64_t backend_features;
>      size_t config_len;
>      void *config;
>      uint16_t config_vector;
>