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

[Y Song <ys114321@gmail.com>]

On Wed, Jul 3, 2019 at 1:51 PM Stanislav Fomichev <sdf@google.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@google.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@linux.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?