Date: Mon, 9 Oct 2023 02:57:35 -0700
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Akihiko Odaki <akihiko.odaki@...nix.com>
Cc: Jason Wang <jasowang@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org, 
	bpf@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, kafai@...com, songliubraving@...com, 
	yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org, 
	rdunlap@...radead.org, willemb@...gle.com, gustavoars@...nel.org, 
	herbert@...dor.apana.org.au, steffen.klassert@...unet.com, nogikh@...gle.com, 
	pablo@...filter.org, decui@...rosoft.com, jakub@...udflare.com, 
	elver@...gle.com, pabeni@...hat.com, 
	Yuri Benditovich <yuri.benditovich@...nix.com>
Subject: Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature

On Mon, Oct 9, 2023 at 3:57 AM Akihiko Odaki <akihiko.odaki@...nix.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@...nix.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@...nix.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@...nix.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@...nix.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.

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