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Date: Tue, 16 Aug 2022 12:26:39 +0800 From: "Zhu, Lingshan" <lingshan.zhu@...el.com> To: Si-Wei Liu <si-wei.liu@...cle.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 8/16/2022 9:58 AM, 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 > https://www.rfc-editor.org/rfc/rfc894.txt I have seen https://www.rfc-editor.org/errata_search.php?rfc=894&rec_status=0, so I think we should return nothing if _F_MTU not set. Thanks >> >> 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. >> >>> + */ >>> + 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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