Date: Mon, 9 Oct 2023 19:12:51 +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 19:06, Willem de Bruijn wrote:
> 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.

I see. I described an alternative to move the bit to cb[] and clear it 
in all code paths that leads to ndo_start_xmit() earlier. Does that 
sound good to you?

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