Re: [PATCH bpf-next v1 8/8] bpf/selftests: Test for bpf_per_cpu_ptr()

[Andrii Nakryiko]

On Thu, Aug 27, 2020 at 8:42 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> Thanks for taking a look!
>
> On Fri, Aug 21, 2020 at 8:30 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Wed, Aug 19, 2020 at 3:42 PM Hao Luo <haoluo@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > ---
> >
> > Acked-by: Andrii Nakryiko <andriin@xxxxxx>
> >
> [...]
> > >
> > >  __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.

>
> [...]