Date: Mon, 9 Oct 2023 03:06:13 -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 3:02 AM Akihiko Odaki <akihiko.odaki@...nix.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@...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.
>
> 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.

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