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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:   Sun, 8 Oct 2023 21:07:06 +0200
Message-ID: <CAF=yD-LdwcXKK66s5gvJNOH8qCWRt3SvEL-GkkVif=kkOaYGhg@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,
        steffen.klassert@secunet.com, nogikh@google.com,
        pablo@netfilter.org, decui@microsoft.com, 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 7:22=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>
> ---

> +static const struct tun_vnet_hash_cap tun_vnet_hash_cap =3D {
> +       .max_indirection_table_length =3D
> +               TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH,
> +
> +       .types =3D VIRTIO_NET_SUPPORTED_HASH_TYPES
> +};

No need to have explicit capabilities exchange like this? Tun either
supports all or none.

>         case TUNSETSTEERINGEBPF:
> -               ret =3D tun_set_ebpf(tun, &tun->steering_prog, argp);
> +               bpf_ret =3D tun_set_ebpf(tun, &tun->steering_prog, argp);
> +               if (IS_ERR(bpf_ret))
> +                       ret =3D PTR_ERR(bpf_ret);
> +               else if (bpf_ret)
> +                       tun->vnet_hash.flags &=3D ~TUN_VNET_HASH_RSS;

Don't make one feature disable another.

TUNSETSTEERINGEBPF and TUNSETVNETHASH are mutually exclusive
functions. If one is enabled the other call should fail, with EBUSY
for instance.

> +       case TUNSETVNETHASH:
> +               len =3D sizeof(vnet_hash);
> +               if (copy_from_user(&vnet_hash, argp, len)) {
> +                       ret =3D -EFAULT;
> +                       break;
> +               }
> +
> +               if (((vnet_hash.flags & TUN_VNET_HASH_REPORT) &&
> +                    (tun->vnet_hdr_sz < sizeof(struct virtio_net_hdr_v1_=
hash) ||
> +                     !tun_is_little_endian(tun))) ||
> +                    vnet_hash.indirection_table_mask >=3D
> +                    TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH) {
> +                       ret =3D -EINVAL;
> +                       break;
> +               }
> +
> +               argp =3D (u8 __user *)argp + len;
> +               len =3D (vnet_hash.indirection_table_mask + 1) * 2;
> +               if (copy_from_user(vnet_hash_indirection_table, argp, len=
)) {
> +                       ret =3D -EFAULT;
> +                       break;
> +               }
> +
> +               argp =3D (u8 __user *)argp + len;
> +               len =3D virtio_net_hash_key_length(vnet_hash.types);
> +
> +               if (copy_from_user(vnet_hash_key, argp, len)) {
> +                       ret =3D -EFAULT;
> +                       break;
> +               }

Probably easier and less error-prone to define a fixed size control
struct with the max indirection table size.

Btw: please trim the CC: list considerably on future patches.