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

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Message-ID: <CAF=yD-K2MQt4nnfwJrx6h6Nii_rho7j1o6nb_jYaSwcWY45pPw@mail.gmail.com>
Date:   Mon, 9 Oct 2023 03:13:15 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Akihiko Odaki <akihiko.odaki@...nix.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, cai@....pw,
        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 Sun, Oct 8, 2023 at 12:22 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>

> @@ -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

> +                       vnet_hdr_content_sz = sizeof(hdr.v1_hash_hdr);
> +                       ret = virtio_net_hdr_v1_hash_from_skb(skb,
> +                                                             &hdr.v1_hash_hdr,
> +                                                             true,
> +                                                             vlan_hlen,
> +                                                             &vnet_hash);
> +               } else {
> +                       vnet_hdr_content_sz = sizeof(hdr.hdr);
> +                       ret = virtio_net_hdr_from_skb(skb, &hdr.hdr,
> +                                                     tun_is_little_endian(tun),
> +                                                     true, vlan_hlen);
> +               }
> +