Re: [PATCH] test_bpf: Fix testing with CONFIG_BPF_JIT_ALWAYS_ON=y on other arches

[Yonghong Song]

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@xxxxxx>

> 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@xxxxxxxxxxxxx>
> > > ---
> > >    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,