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

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Message-ID: <CAF=yD-K4bCBpUVtDR_cv=bagRL+vM4Rusez+uHFTb4_kR8XkpA@mail.gmail.com>
Date: Mon, 9 Oct 2023 04:54:56 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Akihiko Odaki <akihiko.odaki@...nix.com>
Cc: Jason Wang <jasowang@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org, 
	bpf@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, kafai@...com, songliubraving@...com, 
	yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org, 
	rdunlap@...radead.org, willemb@...gle.com, gustavoars@...nel.org, 
	herbert@...dor.apana.org.au, steffen.klassert@...unet.com, nogikh@...gle.com, 
	pablo@...filter.org, decui@...rosoft.com, cai@....pw, jakub@...udflare.com, 
	elver@...gle.com, pabeni@...hat.com, 
	Yuri Benditovich <yuri.benditovich@...nix.com>
Subject: Re: [RFC PATCH 5/7] tun: Introduce virtio-net hashing feature

On Mon, Oct 9, 2023 at 3:44 AM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>
> On 2023/10/09 17:13, Willem de Bruijn wrote:
> > On Sun, Oct 8, 2023 at 12:22 AM Akihiko Odaki <akihiko.odaki@...nix.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@...nix.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.

It should not be possible to downgrade the hdr_sz once v1 is selected.