Jason Wang
2023-Oct-10 05:45 UTC
[RFC PATCH 5/7] tun: Introduce virtio-net hashing feature
On Tue, Oct 10, 2023 at 9:52?AM Akihiko Odaki <akihiko.odaki at daynix.com> wrote:> > On 2023/10/09 19:44, Willem de Bruijn wrote: > > On Mon, Oct 9, 2023 at 3:12?AM Akihiko Odaki <akihiko.odaki at daynix.com> wrote: > >> > >> On 2023/10/09 19:06, Willem de Bruijn wrote: > >>> On Mon, Oct 9, 2023 at 3:02?AM Akihiko Odaki <akihiko.odaki at daynix.com> wrote: > >>>> > >>>> On 2023/10/09 18:57, Willem de Bruijn wrote: > >>>>> On Mon, Oct 9, 2023 at 3:57?AM Akihiko Odaki <akihiko.odaki at daynix.com> wrote: > >>>>>> > >>>>>> On 2023/10/09 17:04, Willem de Bruijn wrote: > >>>>>>> On Sun, Oct 8, 2023 at 3:46?PM Akihiko Odaki <akihiko.odaki at daynix.com> wrote: > >>>>>>>> > >>>>>>>> On 2023/10/09 5:08, Willem de Bruijn wrote: > >>>>>>>>> On Sun, Oct 8, 2023 at 10:04?PM Akihiko Odaki <akihiko.odaki at daynix.com> wrote: > >>>>>>>>>> > >>>>>>>>>> On 2023/10/09 4:07, Willem de Bruijn wrote: > >>>>>>>>>>> On Sun, Oct 8, 2023 at 7:22?AM Akihiko Odaki <akihiko.odaki at daynix.com> wrote: > >>>>>>>>>>>> > >>>>>>>>>>>> virtio-net have two usage of hashes: one is RSS and another is hash > >>>>>>>>>>>> reporting. Conventionally the hash calculation was done by the VMM. > >>>>>>>>>>>> However, computing the hash after the queue was chosen defeats the > >>>>>>>>>>>> purpose of RSS. > >>>>>>>>>>>> > >>>>>>>>>>>> Another approach is to use eBPF steering program. This approach has > >>>>>>>>>>>> another downside: it cannot report the calculated hash due to the > >>>>>>>>>>>> restrictive nature of eBPF. > >>>>>>>>>>>> > >>>>>>>>>>>> Introduce the code to compute hashes to the kernel in order to overcome > >>>>>>>>>>>> thse challenges. An alternative solution is to extend the eBPF steering > >>>>>>>>>>>> program so that it will be able to report to the userspace, but it makes > >>>>>>>>>>>> little sense to allow to implement different hashing algorithms with > >>>>>>>>>>>> eBPF since the hash value reported by virtio-net is strictly defined by > >>>>>>>>>>>> the specification. > >>>>>>>>>>>> > >>>>>>>>>>>> The hash value already stored in sk_buff is not used and computed > >>>>>>>>>>>> independently since it may have been computed in a way not conformant > >>>>>>>>>>>> with the specification. > >>>>>>>>>>>> > >>>>>>>>>>>> Signed-off-by: Akihiko Odaki <akihiko.odaki at daynix.com> > >>>>>>>>>>>> --- > >>>>>>>>>>> > >>>>>>>>>>>> +static const struct tun_vnet_hash_cap tun_vnet_hash_cap = { > >>>>>>>>>>>> + .max_indirection_table_length > >>>>>>>>>>>> + TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH, > >>>>>>>>>>>> + > >>>>>>>>>>>> + .types = VIRTIO_NET_SUPPORTED_HASH_TYPES > >>>>>>>>>>>> +}; > >>>>>>>>>>> > >>>>>>>>>>> No need to have explicit capabilities exchange like this? Tun either > >>>>>>>>>>> supports all or none. > >>>>>>>>>> > >>>>>>>>>> tun does not support VIRTIO_NET_RSS_HASH_TYPE_IP_EX, > >>>>>>>>>> VIRTIO_NET_RSS_HASH_TYPE_TCP_EX, and VIRTIO_NET_RSS_HASH_TYPE_UDP_EX. > >>>>>>>>>> > >>>>>>>>>> It is because the flow dissector does not support IPv6 extensions. The > >>>>>>>>>> specification is also vague, and does not tell how many TLVs should be > >>>>>>>>>> consumed at most when interpreting destination option header so I chose > >>>>>>>>>> to avoid adding code for these hash types to the flow dissector. I doubt > >>>>>>>>>> anyone will complain about it since nobody complains for Linux. > >>>>>>>>>> > >>>>>>>>>> I'm also adding this so that we can extend it later. > >>>>>>>>>> max_indirection_table_length may grow for systems with 128+ CPUs, or > >>>>>>>>>> types may have other bits for new protocols in the future. > >>>>>>>>>> > >>>>>>>>>>> > >>>>>>>>>>>> case TUNSETSTEERINGEBPF: > >>>>>>>>>>>> - ret = tun_set_ebpf(tun, &tun->steering_prog, argp); > >>>>>>>>>>>> + bpf_ret = tun_set_ebpf(tun, &tun->steering_prog, argp); > >>>>>>>>>>>> + if (IS_ERR(bpf_ret)) > >>>>>>>>>>>> + ret = PTR_ERR(bpf_ret); > >>>>>>>>>>>> + else if (bpf_ret) > >>>>>>>>>>>> + tun->vnet_hash.flags &= ~TUN_VNET_HASH_RSS; > >>>>>>>>>>> > >>>>>>>>>>> Don't make one feature disable another. > >>>>>>>>>>> > >>>>>>>>>>> TUNSETSTEERINGEBPF and TUNSETVNETHASH are mutually exclusive > >>>>>>>>>>> functions. If one is enabled the other call should fail, with EBUSY > >>>>>>>>>>> for instance. > >>>>>>>>>>> > >>>>>>>>>>>> + case TUNSETVNETHASH: > >>>>>>>>>>>> + len = sizeof(vnet_hash); > >>>>>>>>>>>> + if (copy_from_user(&vnet_hash, argp, len)) { > >>>>>>>>>>>> + ret = -EFAULT; > >>>>>>>>>>>> + break; > >>>>>>>>>>>> + } > >>>>>>>>>>>> + > >>>>>>>>>>>> + if (((vnet_hash.flags & TUN_VNET_HASH_REPORT) && > >>>>>>>>>>>> + (tun->vnet_hdr_sz < sizeof(struct virtio_net_hdr_v1_hash) || > >>>>>>>>>>>> + !tun_is_little_endian(tun))) || > >>>>>>>>>>>> + vnet_hash.indirection_table_mask >> >>>>>>>>>>>> + TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH) { > >>>>>>>>>>>> + ret = -EINVAL; > >>>>>>>>>>>> + break; > >>>>>>>>>>>> + } > >>>>>>>>>>>> + > >>>>>>>>>>>> + argp = (u8 __user *)argp + len; > >>>>>>>>>>>> + len = (vnet_hash.indirection_table_mask + 1) * 2; > >>>>>>>>>>>> + if (copy_from_user(vnet_hash_indirection_table, argp, len)) { > >>>>>>>>>>>> + ret = -EFAULT; > >>>>>>>>>>>> + break; > >>>>>>>>>>>> + } > >>>>>>>>>>>> + > >>>>>>>>>>>> + argp = (u8 __user *)argp + len; > >>>>>>>>>>>> + len = virtio_net_hash_key_length(vnet_hash.types); > >>>>>>>>>>>> + > >>>>>>>>>>>> + if (copy_from_user(vnet_hash_key, argp, len)) { > >>>>>>>>>>>> + ret = -EFAULT; > >>>>>>>>>>>> + break; > >>>>>>>>>>>> + } > >>>>>>>>>>> > >>>>>>>>>>> Probably easier and less error-prone to define a fixed size control > >>>>>>>>>>> struct with the max indirection table size. > >>>>>>>>>> > >>>>>>>>>> I made its size variable because the indirection table and key may grow > >>>>>>>>>> in the future as I wrote above. > >>>>>>>>>> > >>>>>>>>>>> > >>>>>>>>>>> Btw: please trim the CC: list considerably on future patches. > >>>>>>>>>> > >>>>>>>>>> I'll do so in the next version with the TUNSETSTEERINGEBPF change you > >>>>>>>>>> proposed. > >>>>>>>>> > >>>>>>>>> To be clear: please don't just resubmit with that one change. > >>>>>>>>> > >>>>>>>>> The skb and cb issues are quite fundamental issues that need to be resolved. > >>>>>>>>> > >>>>>>>>> I'd like to understand why adjusting the existing BPF feature for this > >>>>>>>>> exact purpose cannot be amended to return the key it produced. > >>>>>>>> > >>>>>>>> eBPF steering program is not designed for this particular problem in my > >>>>>>>> understanding. It was introduced to derive hash values with an > >>>>>>>> understanding of application-specific semantics of packets instead of > >>>>>>>> generic IP/TCP/UDP semantics. > >>>>>>>> > >>>>>>>> This problem is rather different in terms that the hash derivation is > >>>>>>>> strictly defined by virtio-net. I don't think it makes sense to > >>>>>>>> introduce the complexity of BPF when you always run the same code. > >>>>>>>> > >>>>>>>> It can utilize the existing flow dissector and also make it easier to > >>>>>>>> use for the userspace by implementing this in the kernel. > >>>>>>> > >>>>>>> Ok. There does appear to be overlap in functionality. But it might be > >>>>>>> easier to deploy to just have standard Toeplitz available without > >>>>>>> having to compile and load an eBPF program. > >>>>>>> > >>>>>>> As for the sk_buff and cb[] changes. The first is really not needed. > >>>>>>> sk_buff simply would not scale if every edge case needs a few bits. > >>>>>> > >>>>>> An alternative is to move the bit to cb[] and clear it for every code > >>>>>> paths that lead to ndo_start_xmit(), but I'm worried that it is error-prone. > >>>>>> > >>>>>> I think we can put the bit in sk_buff for now. We can implement the > >>>>>> alternative when we are short of bits. > >>>>> > >>>>> I disagree. sk_buff fields add a cost to every code path. They cannot > >>>>> be added for every edge case. > >>>> > >>>> It only takes an unused bit and does not grow the sk_buff size so I > >>>> think it has practically no cost for now. > >>> > >>> The problem is that that thinking leads to death by a thousand cuts. > >>> > >>> "for now" forces the cost of having to think hard how to avoid growing > >>> sk_buff onto the next person. Let's do it right from the start. > >> > >> I see. I described an alternative to move the bit to cb[] and clear it > >> in all code paths that leads to ndo_start_xmit() earlier. Does that > >> sound good to you? > > > > If you use the control block to pass information between > > __dev_queue_xmit on the tun device and tun_net_xmit, using gso_skb_cb, > > the field can be left undefined in all non-tun paths. tun_select_queue > > can initialize. > > The problem is that tun_select_queue() is not always called. > netdev_core_pick_tx() ensures dev->real_num_tx_queues != 1 before > calling it, but this variable may change later and result in a race > condition. Another case is that XDP with predefined queue. > > > > > I would still use skb->hash to encode the hash. That hash type of that > > field is not strictly defined. It can be siphash from ___skb_get_hash > > or a device hash, which most likely also uses Toeplitz. Then you also > > don't run into the problem of growing the struct size. > > I'm concerned exactly because it's not strictly defined. Someone may > decide to overwrite it later if we are not cautious enough. qdisc_skb_cb > also has sufficient space to contain both of the hash value and type.How about using skb extensions? Thanks>