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

[Akihiko Odaki]

On 2023/10/09 19:07, Willem de Bruijn wrote:
> On Mon, Oct 9, 2023 at 3:05 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> > On 2023/10/09 18:54, Willem de Bruijn wrote:
> > > On Mon, Oct 9, 2023 at 3:44 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> 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.
> > >
> > > It should not be possible to downgrade the hdr_sz once v1 is selected.
> >
> > I see nothing that prevents shrinking the header size.
> >
> > tun->vnet_hash.flags is read after vnet_hdr_sz so the race can happen
> > even for the case the header size grows though this can be fixed by
> > reordering the two reads.
>
> One option is to fail any control path that tries to re-negotiate
> header size once this hash option is enabled?
>
> There is no practical reason to allow feature re-negotiation at any
> arbitrary time.

I think it's a bit awkward interface design since tun allows to 
reconfigure any of its parameters, but it's certainly possible.