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Date: Mon, 24 Aug 2020 18:03:31 -0700 From: Hao Luo <haoluo@...gle.com> To: Andrii Nakryiko <andrii.nakryiko@...il.com> Cc: Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>, "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, Shuah Khan <shuah@...nel.org>, Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andriin@...com>, Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>, John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...omium.org>, Quentin Monnet <quentin@...valent.com>, Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...hat.com>, Andrey Ignatov <rdna@...com>, Jakub Sitnicki <jakub@...udflare.com> Subject: Re: [PATCH bpf-next v1 6/8] bpf: Introduce bpf_per_cpu_ptr() On Sat, Aug 22, 2020 at 12:55 AM Andrii Nakryiko <andrii.nakryiko@...il.com> wrote: > > On Sat, Aug 22, 2020 at 12:49 AM Hao Luo <haoluo@...gle.com> wrote: > > > > On Fri, Aug 21, 2020 at 8:31 PM Andrii Nakryiko > > <andrii.nakryiko@...il.com> wrote: > > > > > > On Fri, Aug 21, 2020 at 8:26 PM Andrii Nakryiko > > > <andrii.nakryiko@...il.com> wrote: > > > > > > > > On Wed, Aug 19, 2020 at 3:42 PM Hao Luo <haoluo@...gle.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@...gle.com> > > > > > --- > > > > > > > > The logic looks correct, few naming nits, but otherwise: > > > > > > > > Acked-by: Andrii Nakryiko <andriin@...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
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