Re: [PATCH RFC v4 0/9] tun: Introduce virtio-net hashing feature

[Stephen Hemminger]

On Tue, 1 Oct 2024 14:54:29 +0900
Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:

> On 2024/09/30 0:33, Stephen Hemminger wrote:
> > On Sun, 29 Sep 2024 16:10:47 +0900
> > Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:
> >   
> >> On 2024/09/29 11:07, Jason Wang wrote:  
> >>> On Fri, Sep 27, 2024 at 3:51 PM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:  
> >>>>
> >>>> On 2024/09/27 13:31, Jason Wang wrote:  
> >>>>> On Fri, Sep 27, 2024 at 10:11 AM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:  
> >>>>>>
> >>>>>> On 2024/09/25 12:30, Jason Wang wrote:  
> >>>>>>> On Tue, Sep 24, 2024 at 5:01 PM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> 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 is based on context
> >>>>>>>> rewrites, which is in feature freeze. We can adopt kfuncs, but they will
> >>>>>>>> not be UAPIs. We opt to ioctl to align with other relevant UAPIs (KVM
> >>>>>>>> and vhost_net).
> >>>>>>>>     
> >>>>>>>
> >>>>>>> I wonder if we could clone the skb and reuse some to store the hash,
> >>>>>>> then the steering eBPF program can access these fields without
> >>>>>>> introducing full RSS in the kernel?  
> >>>>>>
> >>>>>> I don't get how cloning the skb can solve the issue.
> >>>>>>
> >>>>>> We can certainly implement Toeplitz function in the kernel or even with
> >>>>>> tc-bpf to store a hash value that can be used for eBPF steering program
> >>>>>> and virtio hash reporting. However we don't have a means of storing a
> >>>>>> hash type, which is specific to virtio hash reporting and lacks a
> >>>>>> corresponding skb field.  
> >>>>>
> >>>>> I may miss something but looking at sk_filter_is_valid_access(). It
> >>>>> looks to me we can make use of skb->cb[0..4]?  
> >>>>
> >>>> I didn't opt to using cb. Below is the rationale:
> >>>>
> >>>> cb is for tail call so it means we reuse the field for a different
> >>>> purpose. The context rewrite allows adding a field without increasing
> >>>> the size of the underlying storage (the real sk_buff) so we should add a
> >>>> new field instead of reusing an existing field to avoid confusion.
> >>>>
> >>>> We are however no longer allowed to add a new field. In my
> >>>> understanding, this is because it is an UAPI, and eBPF maintainers found
> >>>> it is difficult to maintain its stability.
> >>>>
> >>>> Reusing cb for hash reporting is a workaround to avoid having a new
> >>>> field, but it does not solve the underlying problem (i.e., keeping eBPF
> >>>> as stable as UAPI is unreasonably hard). In my opinion, adding an ioctl
> >>>> is a reasonable option to keep the API as stable as other virtualization
> >>>> UAPIs while respecting the underlying intention of the context rewrite
> >>>> feature freeze.  
> >>>
> >>> Fair enough.
> >>>
> >>> Btw, I remember DPDK implements tuntap RSS via eBPF as well (probably
> >>> via cls or other). It might worth to see if anything we miss here.  
> >>
> >> Thanks for the information. I wonder why they used cls instead of
> >> steering program. Perhaps it may be due to compatibility with macvtap
> >> and ipvtap, which don't steering program.
> >>
> >> Their RSS implementation looks cleaner so I will improve my RSS
> >> implementation accordingly.
> >>  
> > 
> > DPDK needs to support flow rules. The specific case is where packets
> > are classified by a flow, then RSS is done across a subset of the queues.
> > The support for flow in TUN driver is more academic than useful,
> > I fixed it for current BPF, but doubt anyone is using it really.
> > 
> > A full steering program would be good, but would require much more
> > complexity to take a general set of flow rules then communicate that
> > to the steering program.
> >   
> 
> It reminded me of RSS context and flow filter. Some physical NICs 
> support to use a dedicated RSS context for packets matched with flow 
> filter, and virtio is also gaining corresponding features.
> 
> RSS context: https://github.com/oasis-tcs/virtio-spec/issues/178
> Flow filter: https://github.com/oasis-tcs/virtio-spec/issues/179
> 
> I considered about the possibility of supporting these features with tc 
> instead of adding ioctls to tuntap, but it seems not appropriate for 
> virtualization use case.
> 
> In a virtualization use case, tuntap is configured according to requests 
> of guests, and the code processing these requests need to have minimal 
> permissions for security. This goal is achieved by passing a file 
> descriptor that represents a tuntap from a privileged process (e.g., 
> libvirt) to the process handling guest requests (e.g., QEMU).
> 
> However, tc is configured with rtnetlink, which does not seem to have an 
> interface to delegate a permission for one particular device to another 
> process.
> 
> For now I'll continue working on the current approach that is based on 
> ioctl and lacks RSS context and flow filter features. Eventually they 
> are also likely to require new ioctls if they are to be supported with 
> vhost_net.

The DPDK flow handling (rte_flow) was started by Mellanox and many of
the features are to support what that NIC can do. Would be good to have
a tc way to configure that (or devlink).