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

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Date: Mon, 9 Oct 2023 03:07:27 -0700
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 3:05 AM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>
>
>
> On 2023/10/09 18:54, Willem de Bruijn wrote:
> > 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.
>
> 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.