Re: [PATCH net-next v9 3/6] tun: Introduce virtio-net hash feature

[Jason Wang]

On Mon, Mar 10, 2025 at 11:55 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> On Fri, Mar 7, 2025 at 7:01 PM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> >
> > Hash reporting
> > ==============
> >
> > Allow the guest to reuse the hash value to make receive steering
> > consistent between the host and guest, and to save hash computation.
> >
> > RSS
> > ===
> >
> > RSS is a receive steering algorithm that can be negotiated to use with
> > virtio_net. 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 steering program.
> >
> > Introduce the code to perform RSS 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 I didn't
> > opt for it because extending the current mechanism of eBPF steering
> > program as is because it relies on legacy context rewriting, and
> > introducing kfunc-based eBPF will result in non-UAPI dependency while
> > the other relevant virtualization APIs such as KVM and vhost_net are
> > UAPIs.
> >
> > Signed-off-by: Akihiko Odaki <akihiko.odaki@xxxxxxxxxx>
> > Tested-by: Lei Yang <leiyang@xxxxxxxxxx>
> > ---
> >  Documentation/networking/tuntap.rst |   7 ++
> >  drivers/net/Kconfig                 |   1 +
> >  drivers/net/tap.c                   |  68 ++++++++++++++-
> >  drivers/net/tun.c                   |  98 +++++++++++++++++-----
> >  drivers/net/tun_vnet.h              | 159 ++++++++++++++++++++++++++++++++++--
> >  include/linux/if_tap.h              |   2 +
> >  include/linux/skbuff.h              |   3 +
> >  include/uapi/linux/if_tun.h         |  75 +++++++++++++++++
> >  net/core/skbuff.c                   |   4 +
> >  9 files changed, 386 insertions(+), 31 deletions(-)

[...]

> > + *
> > + * The %TUN_VNET_HASH_REPORT flag set with this ioctl will be effective only
> > + * after calling the %TUNSETVNETHDRSZ ioctl with a number greater than or equal
> > + * to the size of &struct virtio_net_hdr_v1_hash.
>
> So you had a dependency check already for vnet hdr len. I'd still
> suggest to split this into rss and hash as they are separated
> features. Then we can use separate data structure for them instead of
> a container struct.
>

Besides this, I think we still need to add new bits to TUNGETIFF to
let userspace know about the new ability.

Thanks