Re: [PATCH 2/2] Allow VIRTIO_F_IN_ORDER to be negotiated for vdpa devices

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

On Tue, Feb 15, 2022 at 12:52:31PM +0530, Gautam Dawar wrote:
> This patch adds the ability to negotiate VIRTIO_F_IN_ORDER bit
> for vhost-vdpa backend when the underlying device supports this
> feature.
> This would aid in reaping performance benefits with HW devices
> that implement this feature. At the same time, it shouldn't have
> any negative impact as vhost-vdpa backend doesn't involve any
> userspace virtqueue operations.
> 
> Signed-off-by: Gautam Dawar <gdawar@xilinx.com>

Having features that hardware implements but qemu does not
means we can't migrate between them.
So I'd rather see a userspace implementation.

> ---
>  hw/net/virtio-net.c | 10 ++++++++++
>  net/vhost-vdpa.c    |  1 +
>  2 files changed, 11 insertions(+)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index cf8ab0f8af..a1089d06f6 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -3507,11 +3507,21 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
>      nc->rxfilter_notify_enabled = 1;
>  
>     if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> +        uint64_t features = BIT_ULL(VIRTIO_F_IN_ORDER);
>          struct virtio_net_config netcfg = {};
> +
>          memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
>          vhost_net_set_config(get_vhost_net(nc->peer),
>              (uint8_t *)&netcfg, 0, ETH_ALEN, VHOST_SET_CONFIG_TYPE_MASTER);
> +
> +	/*
> +         * For vhost-vdpa, if underlying device supports IN_ORDER feature,
> +         * make it available for negotiation.
> +         */
> +	features = vhost_net_get_features(get_vhost_net(nc->peer), features);
> +	n->host_features |= features;
>      }
> +
>      QTAILQ_INIT(&n->rsc_chains);
>      n->qdev = dev;
>  
> diff --git a/net/vhost-vdpa.c b/net/vhost-vdpa.c
> index 25dd6dd975..2886cba5ec 100644
> --- a/net/vhost-vdpa.c
> +++ b/net/vhost-vdpa.c
> @@ -62,6 +62,7 @@ const int vdpa_feature_bits[] = {
>      VIRTIO_NET_F_CTRL_VQ,
>      VIRTIO_F_IOMMU_PLATFORM,
>      VIRTIO_F_RING_PACKED,
> +    VIRTIO_F_IN_ORDER,
>      VIRTIO_NET_F_RSS,
>      VIRTIO_NET_F_HASH_REPORT,
>      VIRTIO_NET_F_GUEST_ANNOUNCE,
> -- 
> 2.30.1