Re: [RFC PATCH 0/7] Support for virtio-net hash reporting

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

> > >>>>> What it does not give is a type indication, such as
> > >>>>> VIRTIO_NET_HASH_TYPE_TCPv6. I don't understand how this would be used.
> > >>>>> In datapaths where the NIC has already computed the four-tuple hash
> > >>>>> and stored it in skb->hash --the common case for servers--, That type
> > >>>>> field is the only reason to have to compute again.
> > >>>> The problem is there's no guarantee that the packet comes from the NIC,
> > >>>> it could be a simple VM2VM or host2VM packet.
> > >>>>
> > >>>> And even if the packet is coming from the NIC that calculates the hash
> > >>>> there's no guarantee that it's the has that guest want (guest may use
> > >>>> different RSS keys).
> > >>> Ah yes, of course.
> > >>>
> > >>> I would still revisit the need to store a detailed hash_type along with
> > >>> the hash, as as far I can tell that conveys no actionable information
> > >>> to the guest.
> > >>
> > >> Yes, need to figure out its usage. According to [1], it only mention
> > >> that storing has type is a charge of driver. Maybe Yuri can answer this.
> > >>
> > > For the case of Windows VM we can't know how exactly the network stack
> > > uses provided hash data (including hash type). But: different releases
> > > of Windows
> > > enable different hash types (for example UDP hash is enabled only on
> > > Server 2016 and up).
> > >
> > > Indeed the Windows requires a little more from the network adapter/driver
> > > than Linux does.
> > >
> > > The addition of RSS support to virtio specification takes in account
> > > the widest set of
> > > requirements (i.e. Windows one), our initial impression is that this
> > > should be enough also for Linux.
> > >
> > > The NDIS specification in part of RSS is _mandatory_ and there are
> > > certification tests
> > > that check that the driver provides the hash data as expected. All the
> > > high-performance
> > > network adapters have such RSS functionality in the hardware.

Thanks for the context.

If Windows requires the driver to pass the hash-type along with the
hash data, then indeed this will be needed.

If it only requires the device to support a subset of of the possible
types, chosen at init, that would be different and it would be cheaper
for the driver to pass this config to the device one time.

> > > With pre-RSS QEMU (i.e. where the virtio-net device does not indicate
> > > the RSS support)
> > > the virtio-net driver for Windows does all the job related to RSS:
> > > - hash calculation
> > > - hash/hash_type delivery
> > > - reporting each packet on the correct CPU according to RSS settings
> > >
> > > With RSS support in QEMU all the packets always come on a proper CPU and
> > > the driver never needs to reschedule them. The driver still need to
> > > calculate the
> > > hash and report it to Windows. In this case we do the same job twice: the device
> > > (QEMU or eBPF) does calculate the hash and get proper queue/CPU to deliver
> > > the packet. But the hash is not delivered by the device, so the driver needs to
> > > recalculate it and report to the Windows.
> > >
> > > If we add HASH_REPORT support (current set of patches) and the device
> > > indicates this
> > > feature we can avoid hash recalculation in the driver assuming we
> > > receive the correct hash
> > > value and hash type. Otherwise the driver can't know which exactly
> > > hash the device has calculated.
> > >
> > > Please let me know if I did not answer the question.
> >
> >
> > I think I get you. The hash type is also a kind of classification (e.g
> > TCP or UDP). Any possibility that it can be deduced from the driver? (Or
> > it could be too expensive to do that).
> >
> The driver does it today (when the device does not offer any features)
> and of course can continue doing it.
> IMO if the device can't report the data according to the spec it
> should not indicate support for the respective feature (or fallback to
> vhost=off).
> Again, IMO if Linux does not need the exact hash_type we can use (for
> Linux) the way that Willem de Brujin suggested in his patchset:
> - just add VIRTIO_NET_HASH_REPORT_L4 to the spec
> - Linux can use MQ + hash delivery (and use VIRTIO_NET_HASH_REPORT_L4)
> - Linux can use (if makes sense) RSS with VIRTIO_NET_HASH_REPORT_L4 and eBPF
> - Windows gets what it needs + eBPF
> So, everyone has what they need at the respective cost.
>
> Regarding use of skb->cb for hash type:
> Currently, if I'm not mistaken, there are 2 bytes at the end of skb->cb:
> skb->cb is 48 bytes array
> There is skb_gso_cb (14 bytes) at offset SKB_GSO_CB_OFFSET(32)
> Is it possible to use one of these 2 bytes for hash_type?
> If yes, shall we extend the skb_gso_cb and place the 1-bytes hash_type
> in it or just emit compilation error if the skb_gso_cb grows beyond 15
> bytes?

Good catch on segmentation taking place between .ndo_select_queue and
.ndo_start_xmit.

That also means that whatever field in the skb is used, has to be
copied to all segments in skb_segment. Which happens for cb. But this
feature is completely unrelated to the skb_gso_cb type. Perhaps
another field with a real type is more clear. For instance, an
extension to the union with napi_id and sender_cpu, as neither is used
in this egress path with .ndo_select_queue?