Re: [PATCH bpf-next v1 6/8] bpf: Introduce bpf_per_cpu_ptr()

From: Hao Luo <haoluo@google.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>, Shuah Khan <shuah@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andriin@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	Quentin Monnet <quentin@isovalent.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>, Andrey Ignatov <rdna@fb.com>,
	Jakub Sitnicki <jakub@cloudflare.com>
Subject: Re: [PATCH bpf-next v1 6/8] bpf: Introduce bpf_per_cpu_ptr()
Date: Mon, 24 Aug 2020 18:03:31 -0700	[thread overview]
Message-ID: <CA+khW7gR=oCPvs07=XwsBPV2NiY4ne2d6mX9L_dj6fYy5grsyA@mail.gmail.com> (raw)
In-Reply-To: <CAEf4BzZwtpvBSE=00XyHGr2p2OD6X_rwnntwvSZBjGvZUEZKCA@mail.gmail.com>

On Sat, Aug 22, 2020 at 12:55 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Sat, Aug 22, 2020 at 12:49 AM Hao Luo <haoluo@google.com> wrote:
> >
> > On Fri, Aug 21, 2020 at 8:31 PM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> > >
> > > On Fri, Aug 21, 2020 at 8:26 PM Andrii Nakryiko
> > > <andrii.nakryiko@gmail.com> wrote:
> > > >
> > > > On Wed, Aug 19, 2020 at 3:42 PM Hao Luo <haoluo@google.com> wrote:
> > > > >
> > > > > Add bpf_per_cpu_ptr() to help bpf programs access percpu vars.
> > > > > bpf_per_cpu_ptr() has the same semantic as per_cpu_ptr() in the kernel
> > > > > except that it may return NULL. This happens when the cpu parameter is
> > > > > out of range. So the caller must check the returned value.
> > > > >
> > > > > Signed-off-by: Hao Luo <haoluo@google.com>
> > > > > ---
> > > >
> > > > The logic looks correct, few naming nits, but otherwise:
> > > >
> > > > Acked-by: Andrii Nakryiko <andriin@fb.com>
> > > >
> > > > >  include/linux/bpf.h      |  3 ++
> > > > >  include/linux/btf.h      | 11 +++++++
> > > > >  include/uapi/linux/bpf.h | 14 +++++++++
> > > > >  kernel/bpf/btf.c         | 10 -------
> > > > >  kernel/bpf/verifier.c    | 64 ++++++++++++++++++++++++++++++++++++++--
> > > > >  kernel/trace/bpf_trace.c | 18 +++++++++++
> > > > >  6 files changed, 107 insertions(+), 13 deletions(-)
> > [...]
> > >
> > > btw, having bpf_this_cpu_ptr(const void *ptr) seems worthwhile as well, WDYT?
> > >
> >
> > It's probably not a good idea, IMHO. How does it interact with
> > preemption? Should we treat it as __this_cpu_ptr()? If so, I feel it's
> > easy to be misused, if the bpf program is called in a preemptible
> > context.
> >
> > Btw, is bpf programs always called with preemption disabled? How about
> > interrupts? I haven't thought about these questions before but I think
> > they matter as we start to have more ways for bpf programs to interact
> > with the kernel.
>
> non-sleepable BPF is always disabling CPU migration, so there is no
> problem with this_cpu_ptr. For sleepable not sure, but we can disable
> this helper for sleepable BPF programs, if that's a problem.
>

Sounds good. I see there is bpf_get_smp_processor_id() and we are
already doing this. I can add this_cpu_ptr() in v2.

Hao