[bpf-next,v3,4/4] riscv, bpf: Mixing bpf2bpf and tailcalls

Commit Message

Pu Lehui Feb. 1, 2024, 8:33 a.m. UTC

From: Pu Lehui <[e-mail]>

In the current RV64 JIT, if we just don't initialize the TCC in subprog,
the TCC can be propagated from the parent process to the subprocess, but
the TCC of the parent process cannot be restored when the subprocess
exits. Since the RV64 TCC is initialized before saving the callee saved
registers into the stack, we cannot use the callee saved register to
pass the TCC, otherwise the original value of the callee saved register
will be destroyed. So we implemented mixing bpf2bpf and tailcalls
similar to x86_64, i.e. using a non-callee saved register to transfer
the TCC between functions, and saving that register to the stack to
protect the TCC value. At the same time, we also consider the scenario
of mixing trampoline.

Tests test_bpf.ko and test_verifier have passed, as well as the relative
testcases of test_progs*.

Signed-off-by: Pu Lehui <[e-mail]>
---
 arch/riscv/net/bpf_jit.h        |  1 +
 arch/riscv/net/bpf_jit_comp64.c | 88 +++++++++++++--------------------
 2 files changed, 36 insertions(+), 53 deletions(-)

Patch

diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
index 8b35f12a4452..d8be89dadf18 100644
--- a/arch/riscv/net/bpf_jit.h
+++ b/arch/riscv/net/bpf_jit.h
@@ -81,6 +81,7 @@  struct rv_jit_context {
 	int nexentries;
 	unsigned long flags;
 	int stack_size;
+	int tcc_offset;
 };
 
 /* Convert from ninsns to bytes. */
diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index 3516d425c5eb..25cd7808e262 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -13,13 +13,11 @@ 
 #include <asm/patch.h>
 #include "bpf_jit.h"
 
+#define RV_REG_TCC		RV_REG_A6
 #define RV_FENTRY_NINSNS	2
 /* fentry and TCC init insns will be skipped on tailcall */
 #define RV_TAILCALL_OFFSET	((RV_FENTRY_NINSNS + 1) * 4)
 
-#define RV_REG_TCC RV_REG_A6
-#define RV_REG_TCC_SAVED RV_REG_S6 /* Store A6 in S6 if program do calls */
-
 static const int regmap[] = {
 	[BPF_REG_0] =	RV_REG_A5,
 	[BPF_REG_1] =	RV_REG_A0,
@@ -51,14 +49,12 @@  static const int pt_regmap[] = {
 };
 
 enum {
-	RV_CTX_F_SEEN_TAIL_CALL =	0,
 	RV_CTX_F_SEEN_CALL =		RV_REG_RA,
 	RV_CTX_F_SEEN_S1 =		RV_REG_S1,
 	RV_CTX_F_SEEN_S2 =		RV_REG_S2,
 	RV_CTX_F_SEEN_S3 =		RV_REG_S3,
 	RV_CTX_F_SEEN_S4 =		RV_REG_S4,
 	RV_CTX_F_SEEN_S5 =		RV_REG_S5,
-	RV_CTX_F_SEEN_S6 =		RV_REG_S6,
 };
 
 static u8 bpf_to_rv_reg(int bpf_reg, struct rv_jit_context *ctx)
@@ -71,7 +67,6 @@  static u8 bpf_to_rv_reg(int bpf_reg, struct rv_jit_context *ctx)
 	case RV_CTX_F_SEEN_S3:
 	case RV_CTX_F_SEEN_S4:
 	case RV_CTX_F_SEEN_S5:
-	case RV_CTX_F_SEEN_S6:
 		__set_bit(reg, &ctx->flags);
 	}
 	return reg;
@@ -86,7 +81,6 @@  static bool seen_reg(int reg, struct rv_jit_context *ctx)
 	case RV_CTX_F_SEEN_S3:
 	case RV_CTX_F_SEEN_S4:
 	case RV_CTX_F_SEEN_S5:
-	case RV_CTX_F_SEEN_S6:
 		return test_bit(reg, &ctx->flags);
 	}
 	return false;
@@ -102,32 +96,6 @@  static void mark_call(struct rv_jit_context *ctx)
 	__set_bit(RV_CTX_F_SEEN_CALL, &ctx->flags);
 }
 
-static bool seen_call(struct rv_jit_context *ctx)
-{
-	return test_bit(RV_CTX_F_SEEN_CALL, &ctx->flags);
-}
-
-static void mark_tail_call(struct rv_jit_context *ctx)
-{
-	__set_bit(RV_CTX_F_SEEN_TAIL_CALL, &ctx->flags);
-}
-
-static bool seen_tail_call(struct rv_jit_context *ctx)
-{
-	return test_bit(RV_CTX_F_SEEN_TAIL_CALL, &ctx->flags);
-}
-
-static u8 rv_tail_call_reg(struct rv_jit_context *ctx)
-{
-	mark_tail_call(ctx);
-
-	if (seen_call(ctx)) {
-		__set_bit(RV_CTX_F_SEEN_S6, &ctx->flags);
-		return RV_REG_S6;
-	}
-	return RV_REG_A6;
-}
-
 static bool is_32b_int(s64 val)
 {
 	return -(1L << 31) <= val && val < (1L << 31);
@@ -252,10 +220,6 @@  static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx)
 		emit_ld(RV_REG_S5, store_offset, RV_REG_SP, ctx);
 		store_offset -= 8;
 	}
-	if (seen_reg(RV_REG_S6, ctx)) {
-		emit_ld(RV_REG_S6, store_offset, RV_REG_SP, ctx);
-		store_offset -= 8;
-	}
 
 	emit_addi(RV_REG_SP, RV_REG_SP, stack_adjust, ctx);
 	/* Set return value. */
@@ -343,7 +307,6 @@  static void emit_branch(u8 cond, u8 rd, u8 rs, int rvoff,
 static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 {
 	int tc_ninsn, off, start_insn = ctx->ninsns;
-	u8 tcc = rv_tail_call_reg(ctx);
 
 	/* a0: &ctx
 	 * a1: &array
@@ -366,9 +329,11 @@  static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	/* if (--TCC < 0)
 	 *     goto out;
 	 */
-	emit_addi(RV_REG_TCC, tcc, -1, ctx);
+	emit_ld(RV_REG_TCC, ctx->tcc_offset, RV_REG_SP, ctx);
+	emit_addi(RV_REG_TCC, RV_REG_TCC, -1, ctx);
 	off = ninsns_rvoff(tc_ninsn - (ctx->ninsns - start_insn));
 	emit_branch(BPF_JSLT, RV_REG_TCC, RV_REG_ZERO, off, ctx);
+	emit_sd(RV_REG_SP, ctx->tcc_offset, RV_REG_TCC, ctx);
 
 	/* prog = array->ptrs[index];
 	 * if (!prog)
@@ -767,7 +732,7 @@  static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 	int i, ret, offset;
 	int *branches_off = NULL;
 	int stack_size = 0, nregs = m->nr_args;
-	int retval_off, args_off, nregs_off, ip_off, run_ctx_off, sreg_off;
+	int retval_off, args_off, nregs_off, ip_off, run_ctx_off, sreg_off, tcc_off;
 	struct bpf_tramp_links *fentry = &tlinks[BPF_TRAMP_FENTRY];
 	struct bpf_tramp_links *fexit = &tlinks[BPF_TRAMP_FEXIT];
 	struct bpf_tramp_links *fmod_ret = &tlinks[BPF_TRAMP_MODIFY_RETURN];
@@ -812,6 +777,8 @@  static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 	 *
 	 * FP - sreg_off    [ callee saved reg	]
 	 *
+	 * FP - tcc_off     [ tail call count	] BPF_TRAMP_F_TAIL_CALL_CTX
+	 *
 	 *		    [ pads              ] pads for 16 bytes alignment
 	 */
 
@@ -853,6 +820,11 @@  static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 	stack_size += 8;
 	sreg_off = stack_size;
 
+	if (flags & BPF_TRAMP_F_TAIL_CALL_CTX) {
+		stack_size += 8;
+		tcc_off = stack_size;
+	}
+
 	stack_size = round_up(stack_size, 16);
 
 	if (!is_struct_ops) {
@@ -879,6 +851,10 @@  static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 		emit_addi(RV_REG_FP, RV_REG_SP, stack_size, ctx);
 	}
 
+	/* store tail call count */
+	if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
+		emit_sd(RV_REG_FP, -tcc_off, RV_REG_TCC, ctx);
+
 	/* callee saved register S1 to pass start time */
 	emit_sd(RV_REG_FP, -sreg_off, RV_REG_S1, ctx);
 
@@ -932,6 +908,9 @@  static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 
 	if (flags & BPF_TRAMP_F_CALL_ORIG) {
 		restore_args(nregs, args_off, ctx);
+		/* restore TCC to RV_REG_TCC before calling the original function */
+		if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
+			emit_ld(RV_REG_TCC, -tcc_off, RV_REG_FP, ctx);
 		ret = emit_call((const u64)orig_call, true, ctx);
 		if (ret)
 			goto out;
@@ -963,6 +942,9 @@  static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 		ret = emit_call((const u64)__bpf_tramp_exit, true, ctx);
 		if (ret)
 			goto out;
+	} else if (flags & BPF_TRAMP_F_TAIL_CALL_CTX) {
+		/* restore TCC to RV_REG_TCC before calling the original function */
+		emit_ld(RV_REG_TCC, -tcc_off, RV_REG_FP, ctx);
 	}
 
 	if (flags & BPF_TRAMP_F_RESTORE_REGS)
@@ -1455,6 +1437,9 @@  int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 		if (ret < 0)
 			return ret;
 
+		/* restore TCC from stack to RV_REG_TCC */
+		emit_ld(RV_REG_TCC, ctx->tcc_offset, RV_REG_SP, ctx);
+
 		ret = emit_call(addr, fixed_addr, ctx);
 		if (ret)
 			return ret;
@@ -1733,8 +1718,7 @@  void bpf_jit_build_prologue(struct rv_jit_context *ctx)
 		stack_adjust += 8;
 	if (seen_reg(RV_REG_S5, ctx))
 		stack_adjust += 8;
-	if (seen_reg(RV_REG_S6, ctx))
-		stack_adjust += 8;
+	stack_adjust += 8; /* RV_REG_TCC */
 
 	stack_adjust = round_up(stack_adjust, 16);
 	stack_adjust += bpf_stack_adjust;
@@ -1749,7 +1733,8 @@  void bpf_jit_build_prologue(struct rv_jit_context *ctx)
 	 * (TCC) register. This instruction is skipped for tail calls.
 	 * Force using a 4-byte (non-compressed) instruction.
 	 */
-	emit(rv_addi(RV_REG_TCC, RV_REG_ZERO, MAX_TAIL_CALL_CNT), ctx);
+	if (!bpf_is_subprog(ctx->prog))
+		emit(rv_addi(RV_REG_TCC, RV_REG_ZERO, MAX_TAIL_CALL_CNT), ctx);
 
 	emit_addi(RV_REG_SP, RV_REG_SP, -stack_adjust, ctx);
 
@@ -1779,22 +1764,14 @@  void bpf_jit_build_prologue(struct rv_jit_context *ctx)
 		emit_sd(RV_REG_SP, store_offset, RV_REG_S5, ctx);
 		store_offset -= 8;
 	}
-	if (seen_reg(RV_REG_S6, ctx)) {
-		emit_sd(RV_REG_SP, store_offset, RV_REG_S6, ctx);
-		store_offset -= 8;
-	}
+	emit_sd(RV_REG_SP, store_offset, RV_REG_TCC, ctx);
+	ctx->tcc_offset = store_offset;
 
 	emit_addi(RV_REG_FP, RV_REG_SP, stack_adjust, ctx);
 
 	if (bpf_stack_adjust)
 		emit_addi(RV_REG_S5, RV_REG_SP, bpf_stack_adjust, ctx);
 
-	/* Program contains calls and tail calls, so RV_REG_TCC need
-	 * to be saved across calls.
-	 */
-	if (seen_tail_call(ctx) && seen_call(ctx))
-		emit_mv(RV_REG_TCC_SAVED, RV_REG_TCC, ctx);
-
 	ctx->stack_size = stack_adjust;
 }
 
@@ -1807,3 +1784,8 @@  bool bpf_jit_supports_kfunc_call(void)
 {
 	return true;
 }
+
+bool bpf_jit_supports_subprog_tailcalls(void)
+{
+	return true;
+}