bpf, riscv: Modify JIT ctx to support compressed instructions

This patch makes the necessary changes to struct rv_jit_context and to
bpf_int_jit_compile to support compressed riscv (RVC) instructions in
the BPF JIT.

It changes the JIT image to be u16 instead of u32, since RVC instructions
are 2 bytes as opposed to 4.

It also changes ctx->offset and ctx->ninsns to refer to 2-byte
instructions rather than 4-byte ones. The riscv PC is required to be
16-bit aligned with or without RVC, so this is sufficient to refer to
any valid riscv offset.

The code for computing jump offsets in bytes is updated accordingly,
and factored into a new "ninsns_rvoff" function to simplify the code.
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200721025241.8077-2-luke.r.nels@gmail.com

arch/riscv/net/bpf_jit.h

@@ -13,6 +13,11 @@
 #include <linux/filter.h>
 #include <asm/cacheflush.h>
 
+static inline bool rvc_enabled(void)
+{
+	return IS_ENABLED(CONFIG_RISCV_ISA_C);
+}
+
 enum {
 	RV_REG_ZERO =	0,	/* The constant value 0 */
 	RV_REG_RA =	1,	/* Return address */
@@ -50,7 +55,7 @@ enum {
 
 struct rv_jit_context {
 	struct bpf_prog *prog;
-	u32 *insns;		/* RV insns */
+	u16 *insns;		/* RV insns */
 	int ninsns;
 	int epilogue_offset;
 	int *offset;		/* BPF to RV */
@@ -58,6 +63,12 @@ struct rv_jit_context {
 	int stack_size;
 };
 
+/* Convert from ninsns to bytes. */
+static inline int ninsns_rvoff(int ninsns)
+{
+	return ninsns << 1;
+}
+
 struct rv_jit_data {
 	struct bpf_binary_header *header;
 	u8 *image;
@@ -74,8 +85,22 @@ static inline void bpf_flush_icache(void *start, void *end)
 	flush_icache_range((unsigned long)start, (unsigned long)end);
 }
 
+/* Emit a 4-byte riscv instruction. */
 static inline void emit(const u32 insn, struct rv_jit_context *ctx)
 {
+	if (ctx->insns) {
+		ctx->insns[ctx->ninsns] = insn;
+		ctx->insns[ctx->ninsns + 1] = (insn >> 16);
+	}
+
+	ctx->ninsns += 2;
+}
+
+/* Emit a 2-byte riscv compressed instruction. */
+static inline void emitc(const u16 insn, struct rv_jit_context *ctx)
+{
+	BUILD_BUG_ON(!rvc_enabled());
+
 	if (ctx->insns)
 		ctx->insns[ctx->ninsns] = insn;
 
@@ -86,7 +111,7 @@ static inline int epilogue_offset(struct rv_jit_context *ctx)
 {
 	int to = ctx->epilogue_offset, from = ctx->ninsns;
 
-	return (to - from) << 2;
+	return ninsns_rvoff(to - from);
 }
 
 /* Return -1 or inverted cond. */
@@ -149,7 +174,7 @@ static inline int rv_offset(int insn, int off, struct rv_jit_context *ctx)
 	off++; /* BPF branch is from PC+1, RV is from PC */
 	from = (insn > 0) ? ctx->offset[insn - 1] : 0;
 	to = (insn + off > 0) ? ctx->offset[insn + off - 1] : 0;
-	return (to - from) << 2;
+	return ninsns_rvoff(to - from);
 }
 
 /* Instruction formats. */

arch/riscv/net/bpf_jit_comp32.c

@@ -644,7 +644,7 @@ static int emit_branch_r64(const s8 *src1, const s8 *src2, s32 rvoff,
 
 	e = ctx->ninsns;
 	/* Adjust for extra insns. */
-	rvoff -= (e - s) << 2;
+	rvoff -= ninsns_rvoff(e - s);
 	emit_jump_and_link(RV_REG_ZERO, rvoff, true, ctx);
 	return 0;
 }
@@ -713,7 +713,7 @@ static int emit_bcc(u8 op, u8 rd, u8 rs, int rvoff, struct rv_jit_context *ctx)
 	if (far) {
 		e = ctx->ninsns;
 		/* Adjust for extra insns. */
-		rvoff -= (e - s) << 2;
+		rvoff -= ninsns_rvoff(e - s);
 		emit_jump_and_link(RV_REG_ZERO, rvoff, true, ctx);
 	}
 	return 0;
@@ -731,7 +731,7 @@ static int emit_branch_r32(const s8 *src1, const s8 *src2, s32 rvoff,
 
 	e = ctx->ninsns;
 	/* Adjust for extra insns. */
-	rvoff -= (e - s) << 2;
+	rvoff -= ninsns_rvoff(e - s);
 
 	if (emit_bcc(op, lo(rs1), lo(rs2), rvoff, ctx))
 		return -1;
@@ -795,7 +795,7 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	 * if (index >= max_entries)
 	 *   goto out;
 	 */
-	off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
+	off = ninsns_rvoff(tc_ninsn - (ctx->ninsns - start_insn));
 	emit_bcc(BPF_JGE, lo(idx_reg), RV_REG_T1, off, ctx);
 
 	/*
@@ -804,7 +804,7 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	 *   goto out;
 	 */
 	emit(rv_addi(RV_REG_T1, RV_REG_TCC, -1), ctx);
-	off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
+	off = ninsns_rvoff(tc_ninsn - (ctx->ninsns - start_insn));
 	emit_bcc(BPF_JSLT, RV_REG_TCC, RV_REG_ZERO, off, ctx);
 
 	/*
@@ -818,7 +818,7 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	if (is_12b_check(off, insn))
 		return -1;
 	emit(rv_lw(RV_REG_T0, off, RV_REG_T0), ctx);
-	off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
+	off = ninsns_rvoff(tc_ninsn - (ctx->ninsns - start_insn));
 	emit_bcc(BPF_JEQ, RV_REG_T0, RV_REG_ZERO, off, ctx);
 
 	/*
@@ -1214,7 +1214,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 			emit_imm32(tmp2, imm, ctx);
 			src = tmp2;
 			e = ctx->ninsns;
-			rvoff -= (e - s) << 2;
+			rvoff -= ninsns_rvoff(e - s);
 		}
 
 		if (is64)

arch/riscv/net/bpf_jit_comp64.c

@@ -304,14 +304,14 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	if (is_12b_check(off, insn))
 		return -1;
 	emit(rv_lwu(RV_REG_T1, off, RV_REG_A1), ctx);
-	off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
+	off = ninsns_rvoff(tc_ninsn - (ctx->ninsns - start_insn));
 	emit_branch(BPF_JGE, RV_REG_A2, RV_REG_T1, off, ctx);
 
 	/* if (TCC-- < 0)
 	 *     goto out;
 	 */
 	emit(rv_addi(RV_REG_T1, tcc, -1), ctx);
-	off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
+	off = ninsns_rvoff(tc_ninsn - (ctx->ninsns - start_insn));
 	emit_branch(BPF_JSLT, tcc, RV_REG_ZERO, off, ctx);
 
 	/* prog = array->ptrs[index];
@@ -324,7 +324,7 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	if (is_12b_check(off, insn))
 		return -1;
 	emit(rv_ld(RV_REG_T2, off, RV_REG_T2), ctx);
-	off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
+	off = ninsns_rvoff(tc_ninsn - (ctx->ninsns - start_insn));
 	emit_branch(BPF_JEQ, RV_REG_T2, RV_REG_ZERO, off, ctx);
 
 	/* goto *(prog->bpf_func + 4); */
@@ -757,7 +757,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 			e = ctx->ninsns;
 
 			/* Adjust for extra insns */
-			rvoff -= (e - s) << 2;
+			rvoff -= ninsns_rvoff(e - s);
 		}
 
 		if (BPF_OP(code) == BPF_JSET) {
@@ -810,7 +810,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 		e = ctx->ninsns;
 
 		/* Adjust for extra insns */
-		rvoff -= (e - s) << 2;
+		rvoff -= ninsns_rvoff(e - s);
 		emit_branch(BPF_OP(code), rd, rs, rvoff, ctx);
 		break;
 
@@ -831,7 +831,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 		if (!is64 && imm < 0)
 			emit(rv_addiw(RV_REG_T1, RV_REG_T1, 0), ctx);
 		e = ctx->ninsns;
-		rvoff -= (e - s) << 2;
+		rvoff -= ninsns_rvoff(e - s);
 		emit_branch(BPF_JNE, RV_REG_T1, RV_REG_ZERO, rvoff, ctx);
 		break;
 

arch/riscv/net/bpf_jit_core.c

@@ -73,7 +73,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 
 	if (ctx->offset) {
 		extra_pass = true;
-		image_size = sizeof(u32) * ctx->ninsns;
+		image_size = sizeof(*ctx->insns) * ctx->ninsns;
 		goto skip_init_ctx;
 	}
 
@@ -103,7 +103,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 			if (jit_data->header)
 				break;
 
-			image_size = sizeof(u32) * ctx->ninsns;
+			image_size = sizeof(*ctx->insns) * ctx->ninsns;
 			jit_data->header =
 				bpf_jit_binary_alloc(image_size,
 						     &jit_data->image,
@@ -114,7 +114,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 				goto out_offset;
 			}
 
-			ctx->insns = (u32 *)jit_data->image;
+			ctx->insns = (u16 *)jit_data->image;
 			/*
 			 * Now, when the image is allocated, the image can
 			 * potentially shrink more (auipc/jalr -> jal).