netdev - Re: [PATCH bpf-next] selftests/bpf: make verifier loop tests arch independent

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Date:   Mon, 8 Jul 2019 09:13:38 -0700
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Y Song <ys114321@...il.com>
Cc:     Stanislav Fomichev <sdf@...gle.com>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Ilya Leoshkevich <iii@...ux.ibm.com>
Subject: Re: [PATCH bpf-next] selftests/bpf: make verifier loop tests arch
 independent

On 07/03, Y Song wrote:
> On Wed, Jul 3, 2019 at 1:51 PM Stanislav Fomichev <sdf@...gle.com> wrote:
> >
> > Take the first x bytes of pt_regs for scalability tests, there is
> > no real reason we need x86 specific rax.
> >
> > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > ---
> >  tools/testing/selftests/bpf/progs/loop1.c | 3 ++-
> >  tools/testing/selftests/bpf/progs/loop2.c | 3 ++-
> >  tools/testing/selftests/bpf/progs/loop3.c | 3 ++-
> >  3 files changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/testing/selftests/bpf/progs/loop1.c b/tools/testing/selftests/bpf/progs/loop1.c
> > index dea395af9ea9..d530c61d2517 100644
> > --- a/tools/testing/selftests/bpf/progs/loop1.c
> > +++ b/tools/testing/selftests/bpf/progs/loop1.c
> > @@ -14,11 +14,12 @@ SEC("raw_tracepoint/kfree_skb")
> >  int nested_loops(volatile struct pt_regs* ctx)
> >  {
> >         int i, j, sum = 0, m;
> > +       volatile int *any_reg = (volatile int *)ctx;
> >
> >         for (j = 0; j < 300; j++)
> >                 for (i = 0; i < j; i++) {
> >                         if (j & 1)
> > -                               m = ctx->rax;
> > +                               m = *any_reg;
> 
> I agree. ctx->rax here is only to generate some operations, which
> cannot be optimized away by the compiler. dereferencing a volatile
> pointee may just serve that purpose.
> 
> Comparing the byte code generated with ctx->rax and *any_reg, they are
> slightly different. Using *any_reg is slighly worse, but this should
> be still okay for the test.
> 
> >                         else
> >                                 m = j;
> >                         sum += i * m;
> > diff --git a/tools/testing/selftests/bpf/progs/loop2.c b/tools/testing/selftests/bpf/progs/loop2.c
> > index 0637bd8e8bcf..91bb89d901e3 100644
> > --- a/tools/testing/selftests/bpf/progs/loop2.c
> > +++ b/tools/testing/selftests/bpf/progs/loop2.c
> > @@ -14,9 +14,10 @@ SEC("raw_tracepoint/consume_skb")
> >  int while_true(volatile struct pt_regs* ctx)
> >  {
> >         int i = 0;
> > +       volatile int *any_reg = (volatile int *)ctx;
> >
> >         while (true) {
> > -               if (ctx->rax & 1)
> > +               if (*any_reg & 1)
> >                         i += 3;
> >                 else
> >                         i += 7;
> > diff --git a/tools/testing/selftests/bpf/progs/loop3.c b/tools/testing/selftests/bpf/progs/loop3.c
> > index 30a0f6cba080..3a7f12d7186c 100644
> > --- a/tools/testing/selftests/bpf/progs/loop3.c
> > +++ b/tools/testing/selftests/bpf/progs/loop3.c
> > @@ -14,9 +14,10 @@ SEC("raw_tracepoint/consume_skb")
> >  int while_true(volatile struct pt_regs* ctx)
> >  {
> >         __u64 i = 0, sum = 0;
> > +       volatile __u64 *any_reg = (volatile __u64 *)ctx;
> >         do {
> >                 i++;
> > -               sum += ctx->rax;
> > +               sum += *any_reg;
> >         } while (i < 0x100000000ULL);
> >         return sum;
> >  }
> > --
> > 2.22.0.410.gd8fdbe21b5-goog
> 
> Ilya Leoshkevich (iii@...ux.ibm.com, cc'ed) has another patch set
> trying to solve this problem by introducing s360 arch register access
> macros. I guess for now that patch set is not needed any more?
Oh, I missed them. Do they fix the tests for other (non-s360) arches as
well? I was trying to fix the issue by not depending on any arch
specific stuff because the test really doesn't care :-)