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

[Ilya Leoshkevich <iii@linux.ibm.com>]

> Am 08.07.2019 um 18:13 schrieb Stanislav Fomichev <sdf@fomichev.me>:
> 
> On 07/03, Y Song wrote:
>> 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?
> 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.

Best regards,
Ilya