[PATCH 5/5] bpf ppc32: Add instructions for atomic_[cmp]xchg

[Hari Bathini]

This adds two atomic opcodes BPF_XCHG and BPF_CMPXCHG on ppc32, both
of which include the BPF_FETCH flag.  The kernel's atomic_cmpxchg
operation fundamentally has 3 operands, but we only have two register
fields. Therefore the operand we compare against (the kernel's API
calls it 'old') is hard-coded to be BPF_REG_R0. Also, kernel's
atomic_cmpxchg returns the previous value at dst_reg + off. JIT the
same for BPF too with return value put in BPF_REG_0.

  BPF_REG_R0 = atomic_cmpxchg(dst_reg + off, BPF_REG_R0, src_reg);

Signed-off-by: Hari Bathini <hbathini at linux.ibm.com>
---
 arch/powerpc/net/bpf_jit_comp32.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
index 5604ae1b60ab..4690fd6e9e52 100644
--- a/arch/powerpc/net/bpf_jit_comp32.c
+++ b/arch/powerpc/net/bpf_jit_comp32.c
@@ -829,6 +829,23 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 				/* we're done if this succeeded */
 				PPC_BCC_SHORT(COND_NE, tmp_idx);
 				break;
+			case BPF_CMPXCHG:
+				/* Compare with old value in BPF_REG_0 */
+				EMIT(PPC_RAW_CMPW(bpf_to_ppc(BPF_REG_0), _R0));
+				/* Don't set if different from old value */
+				PPC_BCC_SHORT(COND_NE, (ctx->idx + 3) * 4);
+				fallthrough;
+			case BPF_XCHG:
+				/* store new value */
+				EMIT(PPC_RAW_STWCX(src_reg, tmp_reg, dst_reg));
+				PPC_BCC_SHORT(COND_NE, tmp_idx);
+				/*
+				 * Return old value in src_reg for BPF_XCHG &
+				 * BPF_REG_0 for BPF_CMPXCHG.
+				 */
+				EMIT(PPC_RAW_MR(imm == BPF_XCHG ? src_reg : bpf_to_ppc(BPF_REG_0),
+						_R0));
+				break;
 			default:
 				pr_err_ratelimited("eBPF filter atomic op code %02x (@%d) unsupported\n",
 						   code, i);
-- 
2.35.1