Linux Virtualization - Oct 2023 - [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature

On Sun, Oct 8, 2023 at 7:22?AM Akihiko Odaki <akihiko.odaki at 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 at daynix.com>
> ---
> +static const struct tun_vnet_hash_cap tun_vnet_hash_cap = {
> +       .max_indirection_table_length > +              
TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH,
> +
> +       .types = VIRTIO_NET_SUPPORTED_HASH_TYPES
> +};
No need to have explicit capabilities exchange like this? Tun either
supports all or none.
>         case TUNSETSTEERINGEBPF:
> -               ret = tun_set_ebpf(tun, &tun->steering_prog, argp);
> +               bpf_ret = tun_set_ebpf(tun, &tun->steering_prog,
argp);
> +               if (IS_ERR(bpf_ret))
> +                       ret = PTR_ERR(bpf_ret);
> +               else if (bpf_ret)
> +                       tun->vnet_hash.flags &= ~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 = sizeof(vnet_hash);
> +               if (copy_from_user(&vnet_hash, argp, len)) {
> +                       ret = -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 >> +           
TUN_VNET_HASH_MAX_INDIRECTION_TABLE_LENGTH) {
> +                       ret = -EINVAL;
> +                       break;
> +               }
> +
> +               argp = (u8 __user *)argp + len;
> +               len = (vnet_hash.indirection_table_mask + 1) * 2;
> +               if (copy_from_user(vnet_hash_indirection_table, argp, len))
{
> +                       ret = -EFAULT;
> +                       break;
> +               }
> +
> +               argp = (u8 __user *)argp + len;
> +               len = virtio_net_hash_key_length(vnet_hash.types);
> +
> +               if (copy_from_user(vnet_hash_key, argp, len)) {
> +                       ret = -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.