Michael S. Tsirkin
2023-Oct-09 11:50 UTC
[RFC PATCH 5/7] tun: Introduce virtio-net hashing feature
On Mon, Oct 09, 2023 at 05:44:20PM +0900, Akihiko Odaki wrote:> On 2023/10/09 17:13, Willem de Bruijn wrote: > > On Sun, Oct 8, 2023 at 12:22?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> > > > > > @@ -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 > > There is a potential race since tun->vnet_hash.flags and vnet_hdr_sz are not > read at once.And then it's a complete mess and you get inconsistent behaviour with packets getting sent all over the place, right? So maybe keep a pointer to this struct so it can be changed atomically then. Maybe even something with rcu I donnu. -- MST