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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Tue, 1 Sep 2020 11:12:26 -0700
Message-ID: <CAEf4Bza5+m72JQ1Q3a2GRetGB7C-Zemvd-ib0u_VKC2nrYkgdQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next v1 8/8] bpf/selftests: Test for bpf_per_cpu_ptr()
To:     Hao Luo <haoluo@google.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>
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On Thu, Aug 27, 2020 at 8:42 PM Hao Luo <haoluo@google.com> wrote:
>
> Thanks for taking a look!
>
> On Fri, Aug 21, 2020 at 8:30 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Wed, Aug 19, 2020 at 3:42 PM Hao Luo <haoluo@google.com> wrote:
> > >
> > > Test bpf_per_cpu_ptr(). Test two paths in the kernel. If the base
> > > pointer points to a struct, the returned reg is of type PTR_TO_BTF_ID.
> > > Direct pointer dereference can be applied on the returned variable.
> > > If the base pointer isn't a struct, the returned reg is of type
> > > PTR_TO_MEM, which also supports direct pointer dereference.
> > >
> > > Signed-off-by: Hao Luo <haoluo@google.com>
> > > ---
> >
> > Acked-by: Andrii Nakryiko <andriin@fb.com>
> >
> [...]
> > >
> > >  __u64 out__runqueues = -1;
> > >  __u64 out__bpf_prog_active = -1;
> > > +__u32 out__rq_cpu = -1;
> > > +unsigned long out__process_counts = -1;
> >
> > try to not use long for variables, it is 32-bit integer in user-space
> > but always 64-bit in BPF. This causes problems when using skeleton on
> > 32-bit architecture.
> >
>
> Ack. I will use another variable of type 'int' instead.

__u64 is fine as well

>
> > >
> > > -extern const struct rq runqueues __ksym; /* struct type global var. */
> > > +extern const struct rq runqueues __ksym; /* struct type percpu var. */
> > >  extern const int bpf_prog_active __ksym; /* int type global var. */
> > > +extern const unsigned long process_counts __ksym; /* int type percpu var. */
> > >
> > >  SEC("raw_tp/sys_enter")
> > >  int handler(const void *ctx)
> > >  {
> > > +       struct rq *rq;
> > > +       unsigned long *count;
> > > +
> > >         out__runqueues = (__u64)&runqueues;
> > >         out__bpf_prog_active = (__u64)&bpf_prog_active;
> > >
> > > +       rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, 1);
> > > +       if (rq)
> > > +               out__rq_cpu = rq->cpu;
> >
> > this is awesome!
> >
> > Are there any per-cpu variables that are arrays? Would be nice to test
> > those too.
> >
> >
>
> There are currently per-cpu arrays, but not common. There is a
> 'pmc_prev_left' in arch/x86, I can add that in this test.

arch-specific variables are bad, because selftests will be failing on
other architectures; let's not do this then.

>
> [...]