[CC: Florent, KP] On Mon, Mar 13, 2023 at 7:50 AM Xu Kuohai <xukuohai@xxxxxxxxxx> 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
