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

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

On Tue, Jan 12, 2021 at 11:11 PM Jason Wang <jasowang@redhat.com> wrote:
>
>
> On 2021/1/13 上午7:47, Willem de Bruijn wrote:
> > On Tue, Jan 12, 2021 at 3:29 PM Yuri Benditovich
> > <yuri.benditovich@daynix.com> wrote:
> >> On Tue, Jan 12, 2021 at 9:49 PM Yuri Benditovich
> >> <yuri.benditovich@daynix.com> wrote:
> >>> On Tue, Jan 12, 2021 at 9:41 PM Yuri Benditovich
> >>> <yuri.benditovich@daynix.com> wrote:
> >>>> Existing TUN module is able to use provided "steering eBPF" to
> >>>> calculate per-packet hash and derive the destination queue to
> >>>> place the packet to. The eBPF uses mapped configuration data
> >>>> containing a key for hash calculation and indirection table
> >>>> with array of queues' indices.
> >>>>
> >>>> This series of patches adds support for virtio-net hash reporting
> >>>> feature as defined in virtio specification. It extends the TUN module
> >>>> and the "steering eBPF" as follows:
> >>>>
> >>>> Extended steering eBPF calculates the hash value and hash type, keeps
> >>>> hash value in the skb->hash and returns index of destination virtqueue
> >>>> and the type of the hash. TUN module keeps returned hash type in
> >>>> (currently unused) field of the skb.
> >>>> skb->__unused renamed to 'hash_report_type'.
> >>>>
> >>>> When TUN module is called later to allocate and fill the virtio-net
> >>>> header and push it to destination virtqueue it populates the hash
> >>>> and the hash type into virtio-net header.
> >>>>
> >>>> VHOST driver is made aware of respective virtio-net feature that
> >>>> extends the virtio-net header to report the hash value and hash report
> >>>> type.
> >>> Comment from Willem de Bruijn:
> >>>
> >>> Skbuff fields are in short supply. I don't think we need to add one
> >>> just for this narrow path entirely internal to the tun device.
> >>>
> >> We understand that and try to minimize the impact by using an already
> >> existing unused field of skb.
> > Not anymore. It was repurposed as a flags field very recently.
> >
> > This use case is also very narrow in scope. And a very short path from
> > data producer to consumer. So I don't think it needs to claim scarce
> > bits in the skb.
> >
> > tun_ebpf_select_queue stores the field, tun_put_user reads it and
> > converts it to the virtio_net_hdr in the descriptor.
> >
> > tun_ebpf_select_queue is called from .ndo_select_queue.  Storing the
> > field in skb->cb is fragile, as in theory some code could overwrite
> > that between field between ndo_select_queue and
> > ndo_start_xmit/tun_net_xmit, from which point it is fully under tun
> > control again. But in practice, I don't believe anything does.
> >
> > Alternatively an existing skb field that is used only on disjoint
> > datapaths, such as ingress-only, could be viable.
>
>
> A question here. We had metadata support in XDP for cooperation between
> eBPF programs. Do we have something similar in the skb?
>
> E.g in the RSS, if we want to pass some metadata information between
> eBPF program and the logic that generates the vnet header (either hard
> logic in the kernel or another eBPF program). Is there any way that can
> avoid the possible conflicts of qdiscs?

Not that I am aware of. The closest thing is cb[].

It'll have to aliase a field like that, that is known unused for the given path.

One other approach that has been used within linear call stacks is out
of band. Like percpu variables softnet_data.xmit.more and
mirred_rec_level. But that is perhaps a bit overwrought for this use
case.

> >
> >>> Instead, you could just run the flow_dissector in tun_put_user if the
> >>> feature is negotiated. Indeed, the flow dissector seems more apt to me
> >>> than BPF here. Note that the flow dissector internally can be
> >>> overridden by a BPF program if the admin so chooses.
> >>>
> >> When this set of patches is related to hash delivery in the virtio-net
> >> packet in general,
> >> it was prepared in context of RSS feature implementation as defined in
> >> virtio spec [1]
> >> In case of RSS it is not enough to run the flow_dissector in tun_put_user:
> >> in tun_ebpf_select_queue the TUN calls eBPF to calculate the hash,
> >> hash type and queue index
> >> according to the (mapped) parameters (key, hash types, indirection
> >> table) received from the guest.
> > TUNSETSTEERINGEBPF was added to support more diverse queue selection
> > than the default in case of multiqueue tun. Not sure what the exact
> > use cases are.
> >
> > But RSS is exactly the purpose of the flow dissector. It is used for
> > that purpose in the software variant RPS. The flow dissector
> > implements a superset of the RSS spec, and certainly computes a
> > four-tuple for TCP/IPv6. In the case of RPS, it is skipped if the NIC
> > has already computed a 4-tuple hash.
> >
> > 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.