Re: [PATCH 5/5] MIPS: Add support for eBPF JIT.

[David Daney]

On 05/26/2017 12:09 PM, Daniel Borkmann wrote:
> On 05/26/2017 05:39 PM, David Daney wrote:
> > On 05/26/2017 08:14 AM, Daniel Borkmann wrote:
> > > On 05/26/2017 02:38 AM, David Daney wrote:
> > > > Since the eBPF machine has 64-bit registers, we only support this in
> > > > 64-bit kernels.  As of the writing of this commit log test-bpf is 
> > > > showing:
> > > >
> > > >    test_bpf: Summary: 316 PASSED, 0 FAILED, [308/308 JIT'ed]
> > > >
> > > > All current test cases are successfully compiled.
> > > >
> > > > Signed-off-by: David Daney <david.daney@xxxxxxxxxx>
> > >
> > > Awesome work!
> > >
> > > Did you also manage to run tools/testing/selftests/bpf/ fine with
> > > the JIT enabled?
> >
> > I haven't done that yet, I will before the next revision.
> >
> > > [...]
> > > > +struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> > > > +{
> > > > +    struct jit_ctx ctx;
> > > > +    unsigned int alloc_size;
> > > > +
> > > > +    /* Only 64-bit kernel supports eBPF */
> > > > +    if (!IS_ENABLED(CONFIG_64BIT) || !bpf_jit_enable)
> > >
> > > Isn't this already reflected by the following?
> > >
> > >    select HAVE_EBPF_JIT if (64BIT && !CPU_MICROMIPS)
> >
> > Not exactly.  The eBPF JIT is in the same file as the classic-BPF JIT, 
> > so when HAVE_EBPF_JIT is false this will indeed never be called.  But 
> > the kernel would otherwise contain all the JIT code.
> >
> > By putting in !IS_ENABLED(CONFIG_64BIT) we allow gcc to eliminate all 
> > the dead code when compiling the JITs.
>
> Side-effect would still be that for cBPF you go through the cBPF
> JIT instead of letting the kernel convert all cBPF to eBPF and
> later on go through your eBPF JIT. If you still prefer to have
> everything in one single file and let gcc eliminate dead code
> then you can just do single line change ...
>
> void bpf_jit_compile(struct bpf_prog *fp)
> {
>          struct jit_ctx ctx;
>          unsigned int alloc_size, tmp_idx;
>
>          if (IS_ENABLED(CONFIG_HAVE_EBPF_JIT) || !bpf_jit_enable)
>                  return;

Yes.  In fact I did that for testing.

The cBPF JIT generates smaller code for:

test_bpf: #274 BPF_MAXINSNS: ld_abs+get_processor_id jited:1 44128 PASS

When we attempt to use the eBPF JIT for this, some of the MIPS branch 
instructions cannot reach their targets (+- 32K instructions).  I didn't 
feel like fixing the code generation quite yet to handle branches that 
span more than 32K instructions, so I left the cBPF in place so I could 
claim that all of the test cases were JITed :-)

For the next revision of the patch I will revisit this.

David.

>          [...]
> }
>
> ... and bpf_prog_ebpf_jited() et al wouldn't need to be changed
> in the core, which are used in kallsyms, and kernel will then
> also be able to automatically JIT all of seccomp-BPF and the
> missing cBPF extensions we have through the eBPF JIT w/o extra
> work.