Hi Will, (Sorry for the late reply, was offline for the last 6 days.)
On Mon, May 01, 2017 at 02:57:20AM +0200, Daniel Borkmann wrote:This work adds BPF_XADD for BPF_W/BPF_DW to the arm64 JIT and therefore completes JITing of all BPF instructions, meaning we can thus also remove the 'notyet' label and do not need to fall back to the interpreter when BPF_XADD is used in a program! This now also brings arm64 JIT in line with x86_64, s390x, ppc64, sparc64, where all current eBPF features are supported. BPF_W example from test_bpf: .u.insns_int = { BPF_ALU32_IMM(BPF_MOV, R0, 0x12), BPF_ST_MEM(BPF_W, R10, -40, 0x10), BPF_STX_XADD(BPF_W, R10, R0, -40), BPF_LDX_MEM(BPF_W, R0, R10, -40), BPF_EXIT_INSN(), }, [...] 00000020: 52800247 mov w7, #0x12 // #18 00000024: 928004eb mov x11, #0xffffffffffffffd8 // #-40 00000028: d280020a mov x10, #0x10 // #16 0000002c: b82b6b2a str w10, [x25,x11] // start of xadd mapping: 00000030: 928004ea mov x10, #0xffffffffffffffd8 // #-40 00000034: 8b19014a add x10, x10, x25 00000038: f9800151 prfm pstl1strm, [x10] 0000003c: 885f7d4b ldxr w11, [x10] 00000040: 0b07016b add w11, w11, w7 00000044: 880b7d4b stxr w11, w11, [x10]This form of STXR (where s == t) is CONSTRAINED UNPREDICTABLE per the architecture; you need to use separate registers for the data and the status flag. You might also be interested in the atomic instructions
Thanks! I tried to find some information on this in the reference guide, but seems I must have overlooked something; should have been conservative instead. I will send a fix for it later today.
introduced in ARMv8.1, which includes the LDADD instruction. You can check elf_hwcap & HWCAP_ATOMICS to see if it's supported.
Will take a look on this as well, thanks for letting me know.
Also, did we get a conclusion on the barrier semantics for this? Currently you don't have any here: is that ok?
As a basis I took the disasm back then for the atomic_add() / atomic64_add(), which, iirc, was mapping to ATOMIC_OP() in atomic_ll_sc.h. This should be equivalent to what the interpreter does in __bpf_prog_run() for the insns BPF_STX | BPF_XADD | BPF_W and BPF_STX | BPF_XADD | BPF_DW. Thanks, Daniel
