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

[Akihiko Odaki]

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.

> 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.