[PATCH v1 1/2] LoongArch: BPF: Support internal-only MOV to resolve per-CPU addrs

[George Guo <dongtai.guo@linux.dev>]

From: George Guo <guodongtai@kylinos.cn>

Support the internal-only BPF_MOV instruction that resolves the absolute
address of per-CPU data from its per-CPU offset. This instruction is used
only for internal inlining optimizations between the BPF verifier and the
JITs (e.g. inlining bpf_get_smp_processor_id() and per-CPU map lookups),
and is not exposed to BPF users.

LoongArch keeps the per-CPU offset of the current CPU in $r21
(__my_cpu_offset), so resolving a per-CPU address only requires adding
$r21 to the source register holding the per-CPU offset. Advertise the
capability via bpf_jit_supports_percpu_insn().

Signed-off-by: George Guo <guodongtai@kylinos.cn>
---
 arch/loongarch/include/asm/inst.h |  1 +
 arch/loongarch/net/bpf_jit.c      | 14 ++++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/arch/loongarch/include/asm/inst.h b/arch/loongarch/include/asm/inst.h
index 76b723590023..44fb5ad26d1a 100644
--- a/arch/loongarch/include/asm/inst.h
+++ b/arch/loongarch/include/asm/inst.h
@@ -404,6 +404,7 @@ enum loongarch_gpr {
 	LOONGARCH_GPR_T6,
 	LOONGARCH_GPR_T7,
 	LOONGARCH_GPR_T8,
+	LOONGARCH_GPR_U0 = 21,	/* Kernel per-CPU base register ($r21) */
 	LOONGARCH_GPR_FP = 22,
 	LOONGARCH_GPR_S0 = 23,
 	LOONGARCH_GPR_S1,
diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index 24913dc7f4e8..20d5bf792108 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -728,6 +728,15 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
 			move_reg(ctx, dst, t1);
 			break;
 		}
+		if (insn_is_mov_percpu_addr(insn)) {
+			if (dst != src)
+				move_reg(ctx, dst, src);
+#ifdef CONFIG_SMP
+			/* dst += __my_cpu_offset, held in $r21 */
+			emit_insn(ctx, addd, dst, dst, LOONGARCH_GPR_U0);
+#endif
+			break;
+		}
 		switch (off) {
 		case 0:
 			move_reg(ctx, dst, src);
@@ -2362,6 +2371,11 @@ bool bpf_jit_supports_fsession(void)
 	return true;
 }
 
+bool bpf_jit_supports_percpu_insn(void)
+{
+	return true;
+}
+
 /* Indicate the JIT backend supports mixing bpf2bpf and tailcalls. */
 bool bpf_jit_supports_subprog_tailcalls(void)
 {
-- 
2.25.1