Re: [RFC PATCH v3 11/11] powerpc64/bpf: Add support for bpf trampolines

Naveen N Rao
2024-07-01T20:00:16Z

On Mon, Jul 01, 2024 at 09:03:52PM GMT, Nicholas Piggin wrote:
> On Fri Jun 21, 2024 at 5:09 AM AEST, Naveen N Rao wrote:
> > Add support for bpf_arch_text_poke() and arch_prepare_bpf_trampoline()
> > for 64-bit powerpc.
>
> What do BPF trampolines give you?

At a very basic level, they provide a way to attach bpf programs at
function entry/exit - as an alternative to ftrace/kprobe - in a more
optimal manner. Commit fec56f5890d9 ("bpf: Introduce BPF trampoline")
has more details.

>
> > BPF prog JIT is extended to mimic 64-bit powerpc approach for ftrace
> > having a single nop at function entry, followed by the function
> > profiling sequence out-of-line and a separate long branch stub for calls
> > to trampolines that are out of range. A dummy_tramp is provided to
> > simplify synchronization similar to arm64.
>
> Synrhonization - between BPF and ftrace interfaces?
>
> > BPF Trampolines adhere to the existing ftrace ABI utilizing a
> > two-instruction profiling sequence, as well as the newer ABI utilizing a
> > three-instruction profiling sequence enabling return with a 'blr'. The
> > trampoline code itself closely follows x86 implementation.
> >
> > While the code is generic, BPF trampolines are only enabled on 64-bit
> > powerpc. 32-bit powerpc will need testing and some updates.
> >
> > Signed-off-by: Naveen N Rao <naveen@kernel.org>
>
> Just a quick glance for now, and I don't know BPF code much.
>
> > ---
> > arch/powerpc/include/asm/ppc-opcode.h | 14 +
> > arch/powerpc/net/bpf_jit.h | 11 +
> > arch/powerpc/net/bpf_jit_comp.c | 702 +++++++++++++++++++++++++-
> > arch/powerpc/net/bpf_jit_comp32.c | 7 +-
> > arch/powerpc/net/bpf_jit_comp64.c | 7 +-
> > 5 files changed, 738 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/ppc-opcode.h b/arch/powerpc/include/asm/ppc-opcode.h
> > index 076ae60b4a55..9eaa2c5d9b73 100644
> > --- a/arch/powerpc/include/asm/ppc-opcode.h
> > +++ b/arch/powerpc/include/asm/ppc-opcode.h
> > @@ -585,12 +585,26 @@
> > #define PPC_RAW_MTSPR(spr, d) (0x7c0003a6 | ___PPC_RS(d) | __PPC_SPR(spr))
> > #define PPC_RAW_EIEIO() (0x7c0006ac)
> >
> > +/* bcl 20,31,$+4 */
> > +#define PPC_RAW_BCL() (0x429f0005)
>
> This is the special bcl form that gives the current address.
> Maybe call it PPC_RAW_BCL4()

Sure.

>
> >
> > +void dummy_tramp(void);
> > +
> > +asm (
> > +" .pushsection .text, \"ax\", @progbits ;"
> > +" .global dummy_tramp ;"
> > +" .type dummy_tramp, @function ;"
> > +"dummy_tramp: ;"
> > +#ifdef CONFIG_FTRACE_PFE_OUT_OF_LINE
> > +" blr ;"
> > +#else
> > +" mflr 11 ;"
>
> Can you just drop this instruction? The caller will always
> have it in r11?

Indeed. Will add a comment and remove the instruction.

>
> > +" mtctr 11 ;"
> > +" mtlr 0 ;"
> > +" bctr ;"
> > +#endif
> > +" .size dummy_tramp, .-dummy_tramp ;"
> > +" .popsection ;"
> > +);
> > +
> > +void bpf_jit_build_fentry_stubs(u32 *image, struct codegen_context *ctx)
> > +{
> > + int ool_stub_idx, long_branch_stub_idx;
> > +
> > + /*
> > + * Out-of-line stub:
> > + * mflr r0
> > + * [b|bl] tramp
> > + * mtlr r0 // only with CONFIG_FTRACE_PFE_OUT_OF_LINE
> > + * b bpf_func + 4
> > + */
> > + ool_stub_idx = ctx->idx;
> > + EMIT(PPC_RAW_MFLR(_R0));
> > + EMIT(PPC_RAW_NOP());
> > + if (IS_ENABLED(CONFIG_FTRACE_PFE_OUT_OF_LINE))
> > + EMIT(PPC_RAW_MTLR(_R0));
> > + WARN_ON_ONCE(!is_offset_in_branch_range(4 - (long)ctx->idx * 4)); /* TODO */
> > + EMIT(PPC_RAW_BRANCH(4 - (long)ctx->idx * 4));
> > +
> > + /*
> > + * Long branch stub:
> > + * .long <dummy_tramp_addr>
> > + * mflr r11
> > + * bcl 20,31,$+4
> > + * mflr r12
> > + * ld r12, -8-SZL(r12)
> > + * mtctr r12
> > + * mtlr r11 // needed to retain ftrace ABI
> > + * bctr
> > + */
>
> You could avoid clobbering LR on >= POWER9 with addpcis instruction. Or
> use a pcrel load with pcrel even. I guess that's something to do later.

Yes, much of BPF JIT could use a re-look to consider opportunities to
emit prefix instructions.

Thanks,
Naveen