Willem de Bruijn
2023-Oct-09 10:44 UTC
[RFC PATCH 5/7] tun: Introduce virtio-net hashing feature
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. 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.