Message-ID: <9b6292f3-9bd5-ecd8-5e42-cd5d12f036e7@oracle.com>
Date:   Tue, 16 Aug 2022 00:58:41 -0700
From:   Si-Wei Liu <si-wei.liu@...cle.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>, jasowang@...hat.com,
        mst@...hat.com
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, parav@...dia.com, xieyongji@...edance.com,
        gautam.dawar@....com
Subject: Re: [PATCH 2/2] vDPA: conditionally read fields in virtio-net dev



On 8/15/2022 6:58 PM, Zhu, Lingshan wrote:
>
>
> On 8/16/2022 7:32 AM, Si-Wei Liu wrote:
>>
>>
>> On 8/15/2022 2:26 AM, 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"
>> Noted there's a typo in the above "The *maximum* length of the data 
>> field of a packet sent over an Ethernet is 1500 octets ..." and the 
>> RFC was written 1984.
> the spec RFC894 says it is 1500, see <a 
> href="https://urldefense.com/v3/__https://www.rfc-editor.org/rfc/rfc894.txt__;!!ACWV5N9M2RV99hQ!MdgxZjw5sp5Qz-GKfwT1IWcw_L4Jo1-UekuJPFz1UrG3YuqirKz7P9ksdJFh1vB6zHJ7z8Q04fpT0-9jWXCtlWM$">https://www.rfc-editor.org/rfc/rfc894.txt</a>
>>
>> Apparently that is no longer true with the introduction of Jumbo size 
>> frame later in the 2000s. I'm not sure what is the point of mention 
>> this ancient RFC. It doesn't say default MTU of any Ethernet 
>> NIC/switch should be 1500 in either  case.
> This could be a larger number for sure, we are trying to find out the 
> min value for Ethernet here, to support 1500 octets, MTU should be 
> 1500 at least, so I assume 1500 should be the default value for MTU
>>
>>>
>>> virtio spec says:"The virtio network device is a virtual ethernet 
>>> card",
>> Right,
>>> so the default MTU value should be 1500 for virtio-net.
>> ... but it doesn't say the default is 1500. At least, not in explicit 
>> way. Why it can't be 1492 or even lower? In practice, if the network 
>> backend has a MTU higher than 1500, there's nothing wrong for guest 
>> to configure default MTU more than 1500.
> same as above
>>
>>>
>>> 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
>>>
>>> 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 <lingshan.zhu@...el.com>
>>> ---
>>>   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;
>> As said, there's no virtio spec defined value for MTU. Please leave 
>> this field out if feature VIRTIO_NET_F_MTU is not negotiated.
> same as above
>>> +    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.
>> You can leave it as a TODO for kernel (vdpa core limitation), but 
>> AFAIK there's nothing userspace needs to do to infer the endianness. 
>> IMHO it's the kernel's job to provide an abstraction rather than rely 
>> on userspace guessing it.
> we have discussed it in another thread, and this comment is suggested 
> by MST.
Can you provide the context or link? It shouldn't work like this, 
otherwise it is breaking uABI. E.g. how will a legacy/BE supporting 
kernel/device be backward compatible with older vdpa tool (which has 
knowledge of this endianness implication/assumption from day one)?

-Siwei

>>
>>> +     */
>>> +    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.
>>> +     */
>>> +
>> Ditto.
> same as above
>
> Thanks
>>
>> Thanks,
>> -Siwei
>>>       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;
>>
>

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