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

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

On 2023/10/09 19:07, Willem de Bruijn wrote:
> On Mon, Oct 9, 2023 at 3:05 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>>
>>
>>
>> On 2023/10/09 18:54, Willem de Bruijn wrote:
>>> On Mon, Oct 9, 2023 at 3:44 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>>>>
>>>> On 2023/10/09 17:13, Willem de Bruijn wrote:
>>>>> On Sun, Oct 8, 2023 at 12:22 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>
>>>>>
>>>>>> @@ -2116,31 +2172,49 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>>>>>>            }
>>>>>>
>>>>>>            if (vnet_hdr_sz) {
>>>>>> -               struct virtio_net_hdr gso;
>>>>>> +               union {
>>>>>> +                       struct virtio_net_hdr hdr;
>>>>>> +                       struct virtio_net_hdr_v1_hash v1_hash_hdr;
>>>>>> +               } hdr;
>>>>>> +               int ret;
>>>>>>
>>>>>>                    if (iov_iter_count(iter) < vnet_hdr_sz)
>>>>>>                            return -EINVAL;
>>>>>>
>>>>>> -               if (virtio_net_hdr_from_skb(skb, &gso,
>>>>>> -                                           tun_is_little_endian(tun), true,
>>>>>> -                                           vlan_hlen)) {
>>>>>> +               if ((READ_ONCE(tun->vnet_hash.flags) & TUN_VNET_HASH_REPORT) &&
>>>>>> +                   vnet_hdr_sz >= sizeof(hdr.v1_hash_hdr) &&
>>>>>> +                   skb->tun_vnet_hash) {
>>>>>
>>>>> Isn't vnet_hdr_sz guaranteed to be >= hdr.v1_hash_hdr, by virtue of
>>>>> the set hash ioctl failing otherwise?
>>>>>
>>>>> Such checks should be limited to control path where possible
>>>>
>>>> There is a potential race since tun->vnet_hash.flags and vnet_hdr_sz are
>>>> not read at once.
>>>
>>> It should not be possible to downgrade the hdr_sz once v1 is selected.
>>
>> I see nothing that prevents shrinking the header size.
>>
>> tun->vnet_hash.flags is read after vnet_hdr_sz so the race can happen
>> even for the case the header size grows though this can be fixed by
>> reordering the two reads.
> 
> One option is to fail any control path that tries to re-negotiate
> header size once this hash option is enabled?
> 
> There is no practical reason to allow feature re-negotiation at any
> arbitrary time.

I think it's a bit awkward interface design since tun allows to 
reconfigure any of its parameters, but it's certainly possible.