Date: Tue, 10 Oct 2023 14:51:17 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Jason Wang <jasowang@...hat.com>
Cc: Willem de Bruijn <willemdebruijn.kernel@...il.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 2023/10/10 14:45, Jason Wang wrote:
> On Tue, Oct 10, 2023 at 9:52 AM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>>
>> On 2023/10/09 19:44, Willem de Bruijn wrote:
>>> On Mon, Oct 9, 2023 at 3:12 AM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>>>>
>>>> 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?
>>>
>>> If you use the control block to pass information between
>>> __dev_queue_xmit on the tun device and tun_net_xmit, using gso_skb_cb,
>>> the field can be left undefined in all non-tun paths. tun_select_queue
>>> can initialize.
>>
>> The problem is that tun_select_queue() is not always called.
>> netdev_core_pick_tx() ensures dev->real_num_tx_queues != 1 before
>> calling it, but this variable may change later and result in a race
>> condition. Another case is that XDP with predefined queue.
>>
>>>
>>> I would still use skb->hash to encode the hash. That hash type of that
>>> field is not strictly defined. It can be siphash from ___skb_get_hash
>>> or a device hash, which most likely also uses Toeplitz. Then you also
>>> don't run into the problem of growing the struct size.
>>
>> I'm concerned exactly because it's not strictly defined. Someone may
>> decide to overwrite it later if we are not cautious enough. qdisc_skb_cb
>> also has sufficient space to contain both of the hash value and type.
> 
> How about using skb extensions?

I think it will work. I'll try it in the next version.

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