linux-kernel - Re: [PATCH RFC bpf-next 0/9] allow HID-BPF to do device IOs

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Date: Fri, 9 Feb 2024 17:26:27 +0100
From: Benjamin Tissoires <benjamin.tissoires@...hat.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Benjamin Tissoires <bentiss@...nel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Jiri Kosina <jikos@...nel.org>, Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>, 
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-input@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC bpf-next 0/9] allow HID-BPF to do device IOs

On Fri, Feb 9, 2024 at 4:42 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Benjamin Tissoires <bentiss@...nel.org> writes:
>
> > [Putting this as a RFC because I'm pretty sure I'm not doing the things
> > correctly at the BPF level.]
> > [Also using bpf-next as the base tree as there will be conflicting
> > changes otherwise]
> >
> > Ideally I'd like to have something similar to bpf_timers, but not
> > in soft IRQ context. So I'm emulating this with a sleepable
> > bpf_tail_call() (see "HID: bpf: allow to defer work in a delayed
> > workqueue").
>
> Why implement a new mechanism? Sounds like what you need is essentially
> the bpf_timer functionality, just running in a different context, right?

Heh, that's exactly why I put in a RFC :)

So yes, the bpf_timer approach is cleaner, but I need it in a
workqueue, as a hrtimer in a softIRQ would prevent me to kzalloc and
wait for the device.

> So why not just add a flag to the timer setup that controls the callback
> context? I've been toying with something similar for restarting XDP TX
> for my queueing patch series (though I'm not sure if this will actually
> end up being needed in the end):
>
> https://git.kernel.org/pub/scm/linux/kernel/git/toke/linux.git/commit/?h=xdp-queueing-08&id=54bc201a358d1ac6ebfe900099315bbd0a76e862
>

Oh, nice. Good idea. But would it be OK to have a "timer-like" where
it actually defers the job in a workqueue instead of using an hrtimer?

I thought I would have to rewrite the entire bpf_timer approach
without the softIRQ, but if I can just add a new flag, that will make
things way simpler for me.

This however raises another issue if I were to use the bpf_timers: now
the HID-BPF kfuncs will not be available as they are only available to
tracing prog types. And when I tried to call them from a bpf_timer (in
softIRQ) they were not available.

Cheers,
Benjamin