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Message-ID: <48e20be1-b658-4117-8856-89ff1df6f48f@daynix.com>
Date:   Mon, 9 Oct 2023 17:44:20 +0900
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Subject: Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature
To:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kselftest@vger.kernel.org, bpf@vger.kernel.org,
        davem@davemloft.net, kuba@kernel.org, ast@kernel.org,
        daniel@iogearbox.net, andrii@kernel.org, kafai@fb.com,
        songliubraving@fb.com, yhs@fb.com, john.fastabend@gmail.com,
        kpsingh@kernel.org, rdunlap@infradead.org, willemb@google.com,
        gustavoars@kernel.org, herbert@gondor.apana.org.au,
        steffen.klassert@secunet.com, nogikh@google.com,
        pablo@netfilter.org, decui@microsoft.com, cai@lca.pw,
        jakub@cloudflare.com, elver@google.com, pabeni@redhat.com,
        Yuri Benditovich <yuri.benditovich@daynix.com>
References: <20231008052101.144422-1-akihiko.odaki@daynix.com>
 <20231008052101.144422-6-akihiko.odaki@daynix.com>
 <CAF=yD-K2MQt4nnfwJrx6h6Nii_rho7j1o6nb_jYaSwcWY45pPw@mail.gmail.com>
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From:   Akihiko Odaki <akihiko.odaki@daynix.com>
In-Reply-To: <CAF=yD-K2MQt4nnfwJrx6h6Nii_rho7j1o6nb_jYaSwcWY45pPw@mail.gmail.com>
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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.