netdev - Re: [PATCH] test_bpf: Fix testing with CONFIG_BPF_JIT_ALWAYS

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Date:   Tue, 20 Mar 2018 10:21:43 -0700
From:   Yonghong Song <yhs@...com>
To:     Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
CC:     <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH] test_bpf: Fix testing with CONFIG_BPF_JIT_ALWAYS_ON=y on
 other arches



On 3/20/18 10:00 AM, Thadeu Lima de Souza Cascardo wrote:
> On Tue, Mar 20, 2018 at 09:05:15AM -0700, Yonghong Song wrote:
>>
>>
>> On 3/20/18 5:58 AM, Thadeu Lima de Souza Cascardo wrote:
>>> Function bpf_fill_maxinsns11 is designed to not be able to be JITed on
>>> x86_64. So, it fails when CONFIG_BPF_JIT_ALWAYS_ON=y, and
>>> commit 09584b406742 ("bpf: fix selftests/bpf test_kmod.sh failure when
>>> CONFIG_BPF_JIT_ALWAYS_ON=y") makes sure that failure is detected on that
>>> case.
>>>
>>> However, it does not fail on other architectures, which have a different
>>> JIT compiler design. So, test_bpf has started to fail to load on those.
>>
>> Here, you mentioned that it did not fail on other architectures. Have you
>> verified all of them or just looked through the algorithm.
> 
>  From our testing, I know at least I get an UNEXPECTED_PASS on arm64, arm, s390x
> and ppc64le. i386 doesn't have JIT, so it doesn't have
> CONFIG_BPF_JIT_ALWAYS_ON=y.
> 
>>
>> Could you give a little bit details about other architectures are okay while
>> x86 is not? Maybe, x86 JIT can be improved some how?
> 
> As the comment on that functions says:
> 
>          /* Hits 70 passes on x86_64, so cannot get JITed there. */
> 
> And looking at x86_64 JIT compiler, you will notice it's looping trying to
> minimize the size of the code, limited to 10 passes. If it does not converge,
> it goes back to the non-JIT code.
> 
> That's not the case on powerpc or arm, that do not do multiple passes. sparc
> seem to do 3 passes, but does not seem to go back to non-JIT code.

Thanks for the explanation.
Reviewed-by: Yonghong Song <yhs@...com>

> 
> Cascardo.
> 
>>
>> Thanks!
>>
>>>
>>> After this fix, test_bpf loads fine on both x86_64 and ppc64el.
>>>
>>> Fixes: 09584b406742 ("bpf: fix selftests/bpf test_kmod.sh failure when CONFIG_BPF_JIT_ALWAYS_ON=y")
>>> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
>>> ---
>>>    lib/test_bpf.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/lib/test_bpf.c b/lib/test_bpf.c
>>> index 2efb213716faa..3e9335493fe49 100644
>>> --- a/lib/test_bpf.c
>>> +++ b/lib/test_bpf.c
>>> @@ -5467,7 +5467,7 @@ static struct bpf_test tests[] = {
>>>    	{
>>>    		"BPF_MAXINSNS: Jump, gap, jump, ...",
>>>    		{ },
>>> -#ifdef CONFIG_BPF_JIT_ALWAYS_ON
>>> +#if defined(CONFIG_BPF_JIT_ALWAYS_ON) && defined(CONFIG_X86)
>>>    		CLASSIC | FLAG_NO_DATA | FLAG_EXPECTED_FAIL,
>>>    #else
>>>    		CLASSIC | FLAG_NO_DATA,
>>>