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From: "Björn Töpel" <bjorn.topel@intel.com>
To: "Toke Høiland-Jørgensen" <toke@redhat.com>,
	"Björn Töpel" <bjorn.topel@gmail.com>,
	ast@kernel.org, daniel@iogearbox.net, netdev@vger.kernel.org,
	bpf@vger.kernel.org
Cc: magnus.karlsson@intel.com, maciej.fijalkowski@intel.com,
	kuba@kernel.org, jonathan.lemon@gmail.com, maximmi@nvidia.com,
	davem@davemloft.net, hawk@kernel.org, john.fastabend@gmail.com,
	ciara.loftus@intel.com, weqaar.a.janjua@intel.com,
	Marek Majtyka <alardam@gmail.com>
Subject: Re: [PATCH bpf-next v2 5/8] libbpf, xsk: select AF_XDP BPF program based on kernel version
Date: Wed, 20 Jan 2021 16:27:12 +0100	[thread overview]
Message-ID: <2751bcd9-b3af-0366-32ee-a52d5919246c@intel.com> (raw)
In-Reply-To: <87eeif4p96.fsf@toke.dk>

On 2021-01-20 16:11, Toke Høiland-Jørgensen wrote:
> Björn Töpel <bjorn.topel@intel.com> writes:
> 
>> On 2021-01-20 14:25, Björn Töpel wrote:
>>> On 2021-01-20 13:52, Toke Høiland-Jørgensen wrote:
>>>> Björn Töpel <bjorn.topel@gmail.com> writes:
>>>>
>>>>> From: Björn Töpel <bjorn.topel@intel.com>
>>>>>
>>>>> Add detection for kernel version, and adapt the BPF program based on
>>>>> kernel support. This way, users will get the best possible performance
>>>>> from the BPF program.
>>>>
>>>> Please do explicit feature detection instead of relying on the kernel
>>>> version number; some distro kernels are known to have a creative notion
>>>> of their own version, which is not really related to the features they
>>>> actually support (I'm sure you know which one I'm referring to ;)).
>>>>
>>>
>>> Right. For a *new* helper, like bpf_redirect_xsk, we rely on rejection
>>> from the verifier to detect support. What about "bpf_redirect_map() now
>>> supports passing return value as flags"? Any ideas how to do that in a
>>> robust, non-version number-based scheme?
>>>
>>
>> Just so that I understand this correctly. Red^WSome distro vendors
>> backport the world, and call that franken kernel, say, 3.10. Is that
>> interpretation correct? My hope was that wasn't the case. :-(
> 
> Yup, indeed. All kernels shipped for the entire lifetime of RHEL8 think
> they are v4.18.0... :/
> 
> I don't think we're the only ones doing it (there are examples in the
> embedded world as well, for instance, and not sure about the other
> enterprise distros), but RHEL is probably the most extreme example.
> 
> We could patch the version check in the distro-supplied version of
> libbpf, of course, but that doesn't help anyone using upstream versions,
> and given the prevalence of vendoring libbpf, I fear that going with the
> version check will just result in a bad user experience...
>

Ok! Thanks for clearing that out!

>> Would it make sense with some kind of BPF-specific "supported
>> features" mechanism? Something else with a bigger scope (whole
>> kernel)?
> 
> Heh, in my opinion, yeah. Seems like we'll finally get it for XDP, but
> for BPF in general the approach has always been probing AFAICT.
> 
> For the particular case of arguments to helpers, I suppose the verifier
> could technically validate value ranges for flags arguments, say. That
> would be nice as an early reject anyway, but I'm not sure if it is
> possible to add after-the-fact without breaking existing programs
> because the verifier can't prove the argument is within the valid range.
> And of course it doesn't help you with compatibility with
> already-released kernels.
>

Hmm, think I have a way forward. I'll use BPF_PROG_TEST_RUN.

If the load fail for the new helper, fallback to bpf_redirect_map(). Use
BPF_PROG_TEST_RUN to make sure that "action via flags" passes.

That should work for you guys as well, right? I'll take a stab at it.


Björn

  reply	other threads:[~2021-01-20 15:32 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-19 15:50 [PATCH bpf-next v2 0/8] Introduce bpf_redirect_xsk() helper Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 1/8] xdp: restructure redirect actions Björn Töpel
2021-01-20 12:44   ` Toke Høiland-Jørgensen
2021-01-20 13:40     ` Björn Töpel
2021-01-20 14:52       ` Toke Høiland-Jørgensen
2021-01-20 15:49         ` Björn Töpel
2021-01-20 16:30           ` Toke Høiland-Jørgensen
2021-01-20 17:26             ` Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 2/8] xsk: remove explicit_free parameter from __xsk_rcv() Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 3/8] xsk: fold xp_assign_dev and __xp_assign_dev Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 4/8] xsk: register XDP sockets at bind(), and add new AF_XDP BPF helper Björn Töpel
2021-01-20  8:25   ` kernel test robot
2021-01-20  8:41     ` Björn Töpel
2021-01-20  8:50   ` kernel test robot
2021-01-20 12:50   ` Toke Høiland-Jørgensen
2021-01-20 13:25     ` Björn Töpel
2021-01-20 14:54       ` Toke Høiland-Jørgensen
2021-01-20 15:18         ` Björn Töpel
2021-01-20 17:29           ` Toke Høiland-Jørgensen
2021-01-20 18:22             ` Björn Töpel
2021-01-20 20:26               ` Toke Høiland-Jørgensen
2021-01-20 21:15                 ` Alexei Starovoitov
2021-01-21  8:18                   ` Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 5/8] libbpf, xsk: select AF_XDP BPF program based on kernel version Björn Töpel
2021-01-20 12:52   ` Toke Høiland-Jørgensen
2021-01-20 13:25     ` Björn Töpel
2021-01-20 14:49       ` Björn Töpel
2021-01-20 15:11         ` Toke Høiland-Jørgensen
2021-01-20 15:27           ` Björn Töpel [this message]
2021-01-20 17:30             ` Toke Høiland-Jørgensen
2021-01-20 18:25             ` Alexei Starovoitov
2021-01-20 18:30               ` Björn Töpel
2021-01-20 14:56       ` Toke Høiland-Jørgensen
2021-01-19 15:50 ` [PATCH bpf-next v2 6/8] libbpf, xsk: select bpf_redirect_xsk(), if supported Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 7/8] selftest/bpf: add XDP socket tests for bpf_redirect_{xsk, map}() Björn Töpel
2021-01-21  7:39   ` Andrii Nakryiko
2021-01-21 12:31     ` Björn Töpel
2021-01-19 15:50 ` [PATCH bpf-next v2 8/8] selftest/bpf: remove a lot of ifobject casting in xdpxceiver Björn Töpel
2021-01-20 13:15 ` [PATCH bpf-next v2 0/8] Introduce bpf_redirect_xsk() helper Maxim Mikityanskiy
2021-01-20 13:27   ` Björn Töpel
2021-01-20 15:57   ` Jesper Dangaard Brouer
2021-01-20 16:19     ` Maciej Fijalkowski
2021-01-21 17:01       ` Jesper Dangaard Brouer
2021-01-22  8:59         ` Magnus Karlsson
2021-01-22  9:45           ` Maciej Fijalkowski

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