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References: <20231008052101.144422-1-akihiko.odaki@daynix.com> <20231008052101.144422-6-akihiko.odaki@daynix.com>
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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Mon, 9 Oct 2023 03:13:15 -0500
Message-ID: <CAF=yD-K2MQt4nnfwJrx6h6Nii_rho7j1o6nb_jYaSwcWY45pPw@mail.gmail.com>
Subject: Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature
To:     Akihiko Odaki <akihiko.odaki@daynix.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,
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        pablo@netfilter.org, decui@microsoft.com, cai@lca.pw,
        jakub@cloudflare.com, elver@google.com, pabeni@redhat.com,
        Yuri Benditovich <yuri.benditovich@daynix.com>
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On Sun, Oct 8, 2023 at 12:22=E2=80=AFAM 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 *tu=
n,
>         }
>
>         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), tr=
ue,
> -                                           vlan_hlen)) {
> +               if ((READ_ONCE(tun->vnet_hash.flags) & TUN_VNET_HASH_REPO=
RT) &&
> +                   vnet_hdr_sz >=3D sizeof(hdr.v1_hash_hdr) &&
> +                   skb->tun_vnet_hash) {

Isn't vnet_hdr_sz guaranteed to be >=3D hdr.v1_hash_hdr, by virtue of
the set hash ioctl failing otherwise?

Such checks should be limited to control path where possible

> +                       vnet_hdr_content_sz =3D sizeof(hdr.v1_hash_hdr);
> +                       ret =3D virtio_net_hdr_v1_hash_from_skb(skb,
> +                                                             &hdr.v1_has=
h_hdr,
> +                                                             true,
> +                                                             vlan_hlen,
> +                                                             &vnet_hash)=
;
> +               } else {
> +                       vnet_hdr_content_sz =3D sizeof(hdr.hdr);
> +                       ret =3D virtio_net_hdr_from_skb(skb, &hdr.hdr,
> +                                                     tun_is_little_endia=
n(tun),
> +                                                     true, vlan_hlen);
> +               }
> +