bpf, riscv: Implement bpf_addr_space_cast instruction

LLVM generates bpf_addr_space_cast instruction while translating
pointers between native (zero) address space and
__attribute__((address_space(N))). The addr_space=0 is reserved as
bpf_arena address space.

rY = addr_space_cast(rX, 0, 1) is processed by the verifier and
converted to normal 32-bit move: wX = wY

rY = addr_space_cast(rX, 1, 0) has to be converted by JIT.
Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Tested-by: Pu Lehui <pulehui@huawei.com>
Reviewed-by: Pu Lehui <pulehui@huawei.com>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/bpf/20240404114203.105970-3-puranjay12@gmail.com

arch/riscv/net/bpf_jit.h

@@ -82,6 +82,7 @@ struct rv_jit_context {
 	unsigned long flags;
 	int stack_size;
 	u64 arena_vm_start;
+	u64 user_vm_start;
 };
 
 /* Convert from ninsns to bytes. */

arch/riscv/net/bpf_jit_comp64.c

@@ -1081,6 +1081,15 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 	/* dst = src */
 	case BPF_ALU | BPF_MOV | BPF_X:
 	case BPF_ALU64 | BPF_MOV | BPF_X:
+		if (insn_is_cast_user(insn)) {
+			emit_mv(RV_REG_T1, rs, ctx);
+			emit_zextw(RV_REG_T1, RV_REG_T1, ctx);
+			emit_imm(rd, (ctx->user_vm_start >> 32) << 32, ctx);
+			emit(rv_beq(RV_REG_T1, RV_REG_ZERO, 4), ctx);
+			emit_or(RV_REG_T1, rd, RV_REG_T1, ctx);
+			emit_mv(rd, RV_REG_T1, ctx);
+			break;
+		}
 		if (imm == 1) {
 			/* Special mov32 for zext */
 			emit_zextw(rd, rd, ctx);
@@ -2024,3 +2033,8 @@ bool bpf_jit_supports_ptr_xchg(void)
 {
 	return true;
 }
+
+bool bpf_jit_supports_arena(void)
+{
+	return true;
+}

arch/riscv/net/bpf_jit_core.c

@@ -81,6 +81,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	}
 
 	ctx->arena_vm_start = bpf_arena_get_kern_vm_start(prog->aux->arena);
+	ctx->user_vm_start = bpf_arena_get_user_vm_start(prog->aux->arena);
 	ctx->prog = prog;
 	ctx->offset = kcalloc(prog->len, sizeof(int), GFP_KERNEL);
 	if (!ctx->offset) {