[RFC] Implementing the BPF dispatcher on ARM64

[Puranjay Mohan]

[CC: Florent, KP]

On Mon, Mar 13, 2023 at 7:50 AM Xu Kuohai <xukuohai at huawei.com> wrote:
>
> [ cc arm list ]
>
> On 3/10/2023 5:33 PM, Puranjay Mohan wrote:
> > Hi,
> > I am starting this thread to know if someone is implementing the BPF
> > dispatcher for ARM64 and if not, what would be needed to make this
> > happen.
> >
> > The basic infra + x86 specific code was introduced in [1] by Björn Töpel.
> >
> > To make BPF dispatcher work on ARM64, the
> > arch_prepare_bpf_dispatcher() has to be implemented in
> > arch/arm64/net/bpf_jit_comp.c.
> >
> > As I am not well versed with XDP and the JIT, I have a few questions
> > regarding this.
> >
> > 1. What is the best way to test this? Is there a selftest that will
> > fail now and will pass once the dispatcher is implemented?
> > 2. As there is no CONFIG_RETPOLINE in ARM64, will the dispatcher be useful.
>
> Hello,
>
> I have some thoughts for bpf dispatcher in arm64.
>
> bpf dispatcher uses static call to convert indirect call instructions to direct
> call instructions, to avoid performance penalty introduced by retpoline. Since
> there is no retpoline or static call in arm64, bpf dispatcher seems useless.
>
> In addition, the range for a direct call instruction in arm64 is +-128MB, but
> jited bpf image address is outside of +-128MB, so it may not be possible to call
> a bpf prog with direct call instruction.

So, to summarize all the information about BPF Dispatcher on ARM64:
1. The range for the B and BL instructions in arm64 is +-128MB, so we
can't use direct jump.
2. Static Calls are not supported on ARM64 yet.
3. bpf_prog_pack allocator for ARM64 is not yet enabled because
bpf_arch_text_copy()
and bpf_arch_text_invalidate() are not implemented.

Even if static calls are implemented the dispatcher can't be
implemented because of point 1.

What would be required to implement bpf_arch_text_copy()
and bpf_arch_text_invalidate(). As enabling the bpf_prog_pack
allocator for ARM64
would be useful in the JIT as well.

>
> >
> > [1] https://github.com/torvalds/linux/commit/75ccbef6369e94ecac696a152a998a978d41376b
> >
>


-- 
Thanks and Regards

Yours Truly,

Puranjay Mohan