[SRU][N/J:linux][PATCH 4/4] powerpc/bpf/64: Add instructions for atomic_[cmp]xchg

[Andrei Gherzan]

BugLink: https://bugs.launchpad.net/bugs/1955011

This adds two atomic opcodes BPF_XCHG and BPF_CMPXCHG on ppc64, 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);

This fixes the test_bpf.sh kernel selftest failure on PPC64 where 74 out
of 577 BPF tests were failing with "FAIL to select_runtime err=-524" due
to unsupported atomic operations.

Signed-off-by: Hari Bathini <hbathini at linux.ibm.com>
Tested-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com> (ppc64le)
Reviewed-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
Link: https://lore.kernel.org/r/20220610155552.25892-4-hbathini@linux.ibm.com
(backported from commit 1e82dfaa7819f03f0b0022be7ca15bbc83090da1)
[agherzan: Context adjustment - used b2p[TMP_REG_x] and b2p[BPF_REG_0]
 notation consistent with existing Ubuntu Jammy code instead of upstream's
 tmp1_reg/tmp2_reg/bpf_to_ppc() local variables and functions. Added a
 compound statement block to scope save_reg and ret_reg variables.]
Signed-off-by: Andrei Gherzan <andrei.gherzan at canonical.com>
---
 arch/powerpc/net/bpf_jit_comp64.c | 40 +++++++++++++++++++++++++++----
 1 file changed, 35 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index 5c55bf41b03e..373218053ab8 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -743,6 +743,10 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 		 */
 		case BPF_STX | BPF_ATOMIC | BPF_W:
 		case BPF_STX | BPF_ATOMIC | BPF_DW:
+		{
+			u32 save_reg = b2p[TMP_REG_2];
+			u32 ret_reg = src_reg;
+
 			/* Get offset into TMP_REG_1 */
 			EMIT(PPC_RAW_LI(b2p[TMP_REG_1], off));
 			tmp_idx = ctx->idx * 4;
@@ -773,6 +777,24 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 			case BPF_XOR | BPF_FETCH:
 				EMIT(PPC_RAW_XOR(b2p[TMP_REG_2], b2p[TMP_REG_2], src_reg));
 				break;
+			case BPF_CMPXCHG:
+				/*
+				 * Return old value in BPF_REG_0 for BPF_CMPXCHG &
+				 * in src_reg for other cases.
+				 */
+				ret_reg = b2p[BPF_REG_0];
+
+				/* Compare with old value in BPF_R0 */
+				if (BPF_SIZE(code) == BPF_DW)
+					EMIT(PPC_RAW_CMPD(b2p[BPF_REG_0], b2p[TMP_REG_2]));
+				else
+					EMIT(PPC_RAW_CMPW(b2p[BPF_REG_0], b2p[TMP_REG_2]));
+				/* Don't set if different from old value */
+				PPC_BCC_SHORT(COND_NE, (ctx->idx + 3) * 4);
+				fallthrough;
+			case BPF_XCHG:
+				save_reg = src_reg;
+				break;
 			default:
 				pr_err_ratelimited(
 					"eBPF filter atomic op code %02x (@%d) unsupported\n",
@@ -782,16 +804,24 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 
 			/* store new value */
 			if (BPF_SIZE(code) == BPF_DW)
-				EMIT(PPC_RAW_STDCX(b2p[TMP_REG_2], b2p[TMP_REG_1], dst_reg));
+				EMIT(PPC_RAW_STDCX(save_reg, b2p[TMP_REG_1], dst_reg));
 			else
-				EMIT(PPC_RAW_STWCX(b2p[TMP_REG_2], b2p[TMP_REG_1], dst_reg));
+				EMIT(PPC_RAW_STWCX(save_reg, b2p[TMP_REG_1], dst_reg));
 			/* we're done if this succeeded */
 			PPC_BCC_SHORT(COND_NE, tmp_idx);
 
-			/* For the BPF_FETCH variant, get old value into src_reg */
-			if (imm & BPF_FETCH)
-				EMIT(PPC_RAW_MR(src_reg, _R0));
+			if (imm & BPF_FETCH) {
+				EMIT(PPC_RAW_MR(ret_reg, _R0));
+				/*
+				 * Skip unnecessary zero-extension for 32-bit cmpxchg.
+				 * For context, see commit 39491867ace5.
+				 */
+				if (BPF_SIZE(code) != BPF_DW && imm == BPF_CMPXCHG &&
+				    insn_is_zext(&insn[i + 1]))
+					addrs[++i] = ctx->idx * 4;
+			}
 			break;
+		}
 
 		/*
 		 * BPF_LDX
-- 
2.43.0