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

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2023/10/09 18:57, Willem de Bruijn wrote:
> On Mon, Oct 9, 2023 at 3:57 AM Akihiko Odaki <akihiko.odaki@daynix.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@daynix.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@daynix.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@daynix.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@daynix.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.