Re: [RFC PATCH 0/6] eBPF RSS support for virtio-net

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Nov 04, 2020 at 10:07:52AM +0800, Jason Wang wrote:
> 
> On 2020/11/3 下午6:32, Yuri Benditovich wrote:
> > 
> > 
> > On Tue, Nov 3, 2020 at 11:02 AM Jason Wang <jasowang@redhat.com
> > <mailto:jasowang@redhat.com>> wrote:
> > 
> > 
> >     On 2020/11/3 上午2:51, Andrew Melnychenko wrote:
> >     > Basic idea is to use eBPF to calculate and steer packets in TAP.
> >     > RSS(Receive Side Scaling) is used to distribute network packets
> >     to guest virtqueues
> >     > by calculating packet hash.
> >     > eBPF RSS allows us to use RSS with vhost TAP.
> >     >
> >     > This set of patches introduces the usage of eBPF for packet steering
> >     > and RSS hash calculation:
> >     > * RSS(Receive Side Scaling) is used to distribute network packets to
> >     > guest virtqueues by calculating packet hash
> >     > * eBPF RSS suppose to be faster than already existing 'software'
> >     > implementation in QEMU
> >     > * Additionally adding support for the usage of RSS with vhost
> >     >
> >     > Supported kernels: 5.8+
> >     >
> >     > Implementation notes:
> >     > Linux TAP TUNSETSTEERINGEBPF ioctl was used to set the eBPF program.
> >     > Added eBPF support to qemu directly through a system call, see the
> >     > bpf(2) for details.
> >     > The eBPF program is part of the qemu and presented as an array
> >     of bpf
> >     > instructions.
> >     > The program can be recompiled by provided Makefile.ebpf(need to
> >     adjust
> >     > 'linuxhdrs'),
> >     > although it's not required to build QEMU with eBPF support.
> >     > Added changes to virtio-net and vhost, primary eBPF RSS is used.
> >     > 'Software' RSS used in the case of hash population and as a
> >     fallback option.
> >     > For vhost, the hash population feature is not reported to the guest.
> >     >
> >     > Please also see the documentation in PATCH 6/6.
> >     >
> >     > I am sending those patches as RFC to initiate the discussions
> >     and get
> >     > feedback on the following points:
> >     > * Fallback when eBPF is not supported by the kernel
> > 
> > 
> >     Yes, and it could also a lacking of CAP_BPF.
> > 
> > 
> >     > * Live migration to the kernel that doesn't have eBPF support
> > 
> > 
> >     Is there anything that we needs special treatment here?
> > 
> > Possible case: rss=on, vhost=on, source system with kernel 5.8
> > (everything works) -> dest. system 5.6 (bpf does not work), the adapter
> > functions, but all the steering does not use proper queues.
> 
> 
> Right, I think we need to disable vhost on dest.
> 
> 
> > 
> > 
> > 
> >     > * Integration with current QEMU build
> > 
> > 
> >     Yes, a question here:
> > 
> >     1) Any reason for not using libbpf, e.g it has been shipped with some
> >     distros
> > 
> > 
> > We intentionally do not use libbpf, as it present only on some distros.
> > We can switch to libbpf, but this will disable bpf if libbpf is not
> > installed
> 
> 
> That's better I think.
> 
> 
> >     2) It would be better if we can avoid shipping bytecodes
> > 
> > 
> > 
> > This creates new dependencies: llvm + clang + ...
> > We would prefer byte code and ability to generate it if prerequisites
> > are installed.
> 
> 
> It's probably ok if we treat the bytecode as a kind of firmware.

That is explicitly *not* OK for inclusion in Fedora. They require that
BPF is compiled from source, and rejected my suggestion that it could
be considered a kind of firmware and thus have an exception from building
from source.

> But in the long run, it's still worthwhile consider the qemu source is used
> for development and llvm/clang should be a common requirement for generating
> eBPF bytecode for host.

So we need to do this right straight way before this merges.

Regards,
Daniel
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