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