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

[Akihiko Odaki]

On 2023/10/09 20:50, Michael S. Tsirkin wrote:
> 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.

I think it's a good idea to use RCU for the vnet_hash members, but 
vnet_hdr_sz is something not specific to vnet_hash so this check will be 
still necessary.