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

[Willem de Bruijn]

On Sun, Oct 8, 2023 at 12:22 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> 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@xxxxxxxxxx>

> @@ -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);
> +               }
> +