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@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
>
> 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
