Subject: Re: [PATCH 2/2] vDPA: conditionally read fields in virtio-net dev

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 15 Aug 2022 11:52:40 -0400

On Mon, Aug 15, 2022 at 05:26:38PM +0800, Zhu Lingshan wrote:
> Some fields of virtio-net device config space are
> conditional on the feature bits, the spec says:
> 
> "The mac address field always exists
> (though is only valid if VIRTIO_NET_F_MAC is set)"
> 
> "max_virtqueue_pairs only exists if VIRTIO_NET_F_MQ
> or VIRTIO_NET_F_RSS is set"
> 
> "mtu only exists if VIRTIO_NET_F_MTU is set"
> 
> so we should read MTU, MAC and MQ in the device config
> space only when these feature bits are offered.
> 
> For MQ, if both VIRTIO_NET_F_MQ and VIRTIO_NET_F_RSS are
> not set, the virtio device should have
> one queue pair as default value, so when userspace querying queue pair numbers,
> it should return mq=1 than zero.
> 
> For MTU, if VIRTIO_NET_F_MTU is not set, we should not read
> MTU from the device config sapce.
> RFC894 <A Standard for the Transmission of IP Datagrams over Ethernet Networks>
> says:"The minimum length of the data field of a packet sent over an
> Ethernet is 1500 octets, thus the maximum length of an IP datagram
> sent over an Ethernet is 1500 octets.  Implementations are encouraged
> to support full-length packets"
> 
> virtio spec says:"The virtio network device is a virtual ethernet card",
> so the default MTU value should be 1500 for virtio-net.
> 
> For MAC, the spec says:"If the VIRTIO_NET_F_MAC feature bit is set,
> the configuration space mac entry indicates the “physical” address
> of the network card, otherwise the driver would typically
> generate a random local MAC address." So there is no
> default MAC address if VIRTIO_NET_F_MAC not set.
> 
> This commits introduces functions vdpa_dev_net_mtu_config_fill()
> and vdpa_dev_net_mac_config_fill() to fill MTU and MAC.
> It also fixes vdpa_dev_net_mq_config_fill() to report correct
> MQ when _F_MQ is not present.
> 
> These functions should check devices features than driver
> features, and struct vdpa_device is not needed as a parameter
> 
> The test & userspace tool output:
> 
> Feature bit VIRTIO_NET_F_MTU, VIRTIO_NET_F_RSS, VIRTIO_NET_F_MQ
> and VIRTIO_NET_F_MAC can be mask out by hardcode.
> 
> However, it is challenging to "disable" the related fields
> in the HW device config space, so let's just assume the values
> are meaningless if the feature bits are not set.
> 
> Before this change, when feature bits for RSS, MQ, MTU and MAC
> are not set, iproute2 output:
> $vdpa vdpa0: mac 00:e8:ca:11:be:05 link up link_announce false mtu 1500
>   negotiated_features

where does it get 1500? what if there's e.g. 0 in the mtu field?

> without this commit, function vdpa_dev_net_config_fill()
> reads all config space fields unconditionally, so let's
> assume the MAC and MTU are meaningless, and it checks
> MQ with driver_features, so we don't see max_vq_pairs.
> 
> After applying this commit, when feature bits for
> MQ, RSS, MAC and MTU are not set,iproute2 output:
> $vdpa dev config show vdpa0
> vdpa0: link up link_announce false max_vq_pairs 1 mtu 1500
>   negotiated_features
> 
> As explained above:
> Here is no MAC, because VIRTIO_NET_F_MAC is not set,
> and there is no default value for MAC. It shows
> max_vq_paris = 1 because even without MQ feature,
> a functional virtio-net must have one queue pair.
> mtu = 1500 is the default value as ethernet
> required.
> 
> This commit also add supplementary comments for
> __virtio16_to_cpu(true, xxx) operations in
> vdpa_dev_net_config_fill() and vdpa_fill_stats_rec()
> 
> Signed-off-by: Zhu Lingshan <[email-masked]>
> ---
>  drivers/vdpa/vdpa.c | 60 +++++++++++++++++++++++++++++++++++----------
>  1 file changed, 47 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
> index efb55a06e961..a74660b98979 100644
> --- a/drivers/vdpa/vdpa.c
> +++ b/drivers/vdpa/vdpa.c
> @@ -801,19 +801,44 @@ static int vdpa_nl_cmd_dev_get_dumpit(struct sk_buff *msg, struct netlink_callba
>  	return msg->len;
>  }
>  
> -static int vdpa_dev_net_mq_config_fill(struct vdpa_device *vdev,
> -				       struct sk_buff *msg, u64 features,
> +static int vdpa_dev_net_mq_config_fill(struct sk_buff *msg, u64 features,
>  				       const struct virtio_net_config *config)
>  {
>  	u16 val_u16;
>  
> -	if ((features & BIT_ULL(VIRTIO_NET_F_MQ)) == 0)
> -		return 0;
> +	if ((features & BIT_ULL(VIRTIO_NET_F_MQ)) == 0 &&
> +	    (features & BIT_ULL(VIRTIO_NET_F_RSS)) == 0)
> +		val_u16 = 1;
> +	else
> +		val_u16 = __virtio16_to_cpu(true, config->max_virtqueue_pairs);
>  
> -	val_u16 = le16_to_cpu(config->max_virtqueue_pairs);
>  	return nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MAX_VQP, val_u16);
>  }
>  
> +static int vdpa_dev_net_mtu_config_fill(struct sk_buff *msg, u64 features,
> +					const struct virtio_net_config *config)
> +{
> +	u16 val_u16;
> +
> +	if ((features & BIT_ULL(VIRTIO_NET_F_MTU)) == 0)
> +		val_u16 = 1500;
> +	else
> +		val_u16 = __virtio16_to_cpu(true, config->mtu);
> +
> +	return nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU, val_u16);
> +}
> +
> +static int vdpa_dev_net_mac_config_fill(struct sk_buff *msg, u64 features,
> +					const struct virtio_net_config *config)
> +{
> +	if ((features & BIT_ULL(VIRTIO_NET_F_MAC)) == 0)
> +		return 0;
> +	else
> +		return  nla_put(msg, VDPA_ATTR_DEV_NET_CFG_MACADDR,
> +				sizeof(config->mac), config->mac);
> +}
> +
> +
>  static int vdpa_dev_net_config_fill(struct vdpa_device *vdev, struct sk_buff *msg)
>  {
>  	struct virtio_net_config config = {};
> @@ -822,18 +847,16 @@ static int vdpa_dev_net_config_fill(struct vdpa_device *vdev, struct sk_buff *ms
>  
>  	vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
>  
> -	if (nla_put(msg, VDPA_ATTR_DEV_NET_CFG_MACADDR, sizeof(config.mac),
> -		    config.mac))
> -		return -EMSGSIZE;
> +	/*
> +	 * Assume little endian for now, userspace can tweak this for
> +	 * legacy guest support.
> +	 */
> +	val_u16 = __virtio16_to_cpu(true, config.status);
>  
>  	val_u16 = __virtio16_to_cpu(true, config.status);
>  	if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_STATUS, val_u16))
>  		return -EMSGSIZE;
>  
> -	val_u16 = __virtio16_to_cpu(true, config.mtu);
> -	if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU, val_u16))
> -		return -EMSGSIZE;
> -
>  	features_driver = vdev->config->get_driver_features(vdev);
>  	if (nla_put_u64_64bit(msg, VDPA_ATTR_DEV_NEGOTIATED_FEATURES, features_driver,
>  			      VDPA_ATTR_PAD))
> @@ -846,7 +869,13 @@ static int vdpa_dev_net_config_fill(struct vdpa_device *vdev, struct sk_buff *ms
>  			      VDPA_ATTR_PAD))
>  		return -EMSGSIZE;
>  
> -	return vdpa_dev_net_mq_config_fill(vdev, msg, features_driver, &config);
> +	if (vdpa_dev_net_mac_config_fill(msg, features_device, &config))
> +		return -EMSGSIZE;
> +
> +	if (vdpa_dev_net_mtu_config_fill(msg, features_device, &config))
> +		return -EMSGSIZE;
> +
> +	return vdpa_dev_net_mq_config_fill(msg, features_device, &config);
>  }
>  
>  static int
> @@ -914,6 +943,11 @@ static int vdpa_fill_stats_rec(struct vdpa_device *vdev, struct sk_buff *msg,
>  	}
>  	vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
>  
> +	/*
> +	 * Assume little endian for now, userspace can tweak this for
> +	 * legacy guest support.
> +	 */
> +
>  	max_vqp = __virtio16_to_cpu(true, config.max_virtqueue_pairs);
>  	if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MAX_VQP, max_vqp))
>  		return -EMSGSIZE;
> -- 
> 2.31.1