Date:   Mon, 9 Oct 2023 17:57:13 +0900
From:   Akihiko Odaki <akihiko.odaki@...nix.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.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 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.

> 
> 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.

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