Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature

[Jason Wang]

On Tue, Oct 10, 2023 at 9:52 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
>
> On 2023/10/09 19:44, Willem de Bruijn wrote:
> > On Mon, Oct 9, 2023 at 3:12 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> >>
> >> On 2023/10/09 19:06, Willem de Bruijn wrote:
> >>> On Mon, Oct 9, 2023 at 3:02 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> >>>>
> >>>> On 2023/10/09 18:57, Willem de Bruijn wrote:
> >>>>> On Mon, Oct 9, 2023 at 3:57 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> >>>>>>
> >>>>>> On 2023/10/09 17:04, Willem de Bruijn wrote:
> >>>>>>> On Sun, Oct 8, 2023 at 3:46 PM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> >>>>>>>>
> >>>>>>>> On 2023/10/09 5:08, Willem de Bruijn wrote:
> >>>>>>>>> On Sun, Oct 8, 2023 at 10:04 PM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> >>>>>>>>>>
> >>>>>>>>>> On 2023/10/09 4:07, Willem de Bruijn wrote:
> >>>>>>>>>>> On Sun, Oct 8, 2023 at 7:22 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> 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@xxxxxxxxxx>
> >>>>>>>>>>>> ---
> >>>>>>>>>>>
> >>>>>>>>>>>> +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

>