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

[Willem de Bruijn]

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.

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.