Willem de Bruijn
2023-Oct-09 09:57 UTC
[RFC PATCH 5/7] tun: Introduce virtio-net hashing feature
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.> > > > > For the control block, generally it is not safe to use that across > > layers. In this case, between qdisc enqueue of a given device and > > ndo_start_xmit of that device, I suppose it is. Though uncommon. I > > wonder if there is any precedent. > > That's one of the reasons modifying qdisc_skb_cb instead of creating > another struct embedding it as bpf_skb_data_end and tc_skb_cb do. This > clarifies that it is qdisc's responsibility to keep these data intact. > > > > > The data will have to be stored in the skb somewhere. A simpler option > > is just skb->hash? This code would use skb_get_hash, if it would > > always produce a Toeplitz hash, anyway. > > We still need tun_vnet_hash_report to identify hash type. > > skb_get_hash() uses Siphash instead of Toeplitz, and the configuration > of Toeplitz relies on tun's internal state. It is still possible to > store a hash calculated with tun's own logic to skb->hash, but someone > may later overwrite it with __skb_get_hash() though it's unlikely.That won't happen in this data path. Fair point on the hash type.