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

From: Stanislav Fomichev <[email-masked]>
Date: Mon, 8 Jul 2019 14:20:12 -0700

On 07/08, Ilya Leoshkevich wrote:
> 
> 
> > Am 08.07.2019 um 18:13 schrieb Stanislav Fomichev <[email-masked]>:
> > 
> > On 07/03, Y Song wrote:
> >> On Wed, Jul 3, 2019 at 1:51 PM Stanislav Fomichev <[email-masked]> 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 <[email-masked]>
> >>> ---
> >>> 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 ([email-masked], 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 :-)
> 
> They are supposed to work for everything that defines PT_REGS_RC in
> bpf_helpers.h, but I have to admit I tested only x86_64 and s390.
> 
> The main source of problems with my approach were mismatching definitions
> of struct pt_regs for userspace and kernel, and because of that there was
> some tweaking required for both arches. I will double check how it looks
> for others (arm, mips, ppc, sparc) tomorrow.
Thanks, I've tested your patches and they fix my issue as well. So you
can have my Tested-by if we'd go with your approach.

One thing I don't understand is: why do you add 'ifdef __KERNEL__' to
the bpf_helpers.h for x86 case? Who is using bpf_helpers.h with
__KERNEL__ defined? Is it perf?

> Best regards,
> Ilya