Date:   Wed, 16 Jan 2019 00:49:58 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Björn Töpel <bjorn.topel@...il.com>,
        linux-riscv@...ts.infradead.org
Cc:     palmer@...ive.com, davidlee@...ive.com, netdev@...r.kernel.org
Subject: Re: [RFC PATCH 3/3] bpf, riscv: added eBPF JIT for RV64G

On 01/15/2019 09:35 AM, Björn Töpel wrote:
> This commit adds eBPF JIT for RV64G.
> 
> Codewise, it needs some refactoring. Currently there's a bit too much
> copy-and-paste going on, and I know some places where I could optimize
> the code generation a bit (mostly BPF_K type of instructions, dealing
> with immediates).

Nice work! :)

> From a features perspective, two things are missing:
> 
> * tail calls
> * "far-branches", i.e. conditional branches that reach beyond 13b.
> 
> The test_bpf.ko passes all tests.

Did you also check test_verifier under jit with/without jit hardening
enabled? That one contains lots of runtime tests as well. Probably makes
sense to check under CONFIG_BPF_JIT_ALWAYS_ON to see what fails the JIT;
the test_verifier also contains various tail call tests targeted at JITs,
for example.

Nit: please definitely also add a MAINTAINERS entry with at least yourself
under BPF JIT section, and update Documentation/sysctl/net.txt with riscv64.

> Signed-off-by: Björn Töpel <bjorn.topel@...il.com>
> ---
>  arch/riscv/net/bpf_jit_comp.c | 1608 +++++++++++++++++++++++++++++++++
>  1 file changed, 1608 insertions(+)
> 
> diff --git a/arch/riscv/net/bpf_jit_comp.c b/arch/riscv/net/bpf_jit_comp.c
> index 7e359d3249ee..562d56eb8d23 100644
> --- a/arch/riscv/net/bpf_jit_comp.c
> +++ b/arch/riscv/net/bpf_jit_comp.c
> @@ -1,4 +1,1612 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * BPF JIT compiler for RV64G
> + *
> + * Copyright(c) 2019 Björn Töpel <bjorn.topel@...il.com>
> + *
> + */
> +
> +#include <linux/bpf.h>
> +#include <linux/filter.h>
> +#include <asm/cacheflush.h>
> +
> +#define TMP_REG_0 (MAX_BPF_JIT_REG + 0)
> +#define TMP_REG_1 (MAX_BPF_JIT_REG + 1)

Not used?

> +#define TAIL_CALL_REG (MAX_BPF_JIT_REG + 2)
> +
> +enum rv_register {
> +	RV_REG_ZERO =	0,	/* The constant value 0 */
> +	RV_REG_RA =	1,	/* Return address */
> +	RV_REG_SP =	2,	/* Stack pointer */
> +	RV_REG_GP =	3,	/* Global pointer */
> +	RV_REG_TP =	4,	/* Thread pointer */
> +	RV_REG_T0 =	5,	/* Temporaries */
> +	RV_REG_T1 =	6,
> +	RV_REG_T2 =	7,
> +	RV_REG_FP =	8,
> +	RV_REG_S1 =	9,	/* Saved registers */
> +	RV_REG_A0 =	10,	/* Function argument/return values */
> +	RV_REG_A1 =	11,	/* Function arguments */
> +	RV_REG_A2 =	12,
> +	RV_REG_A3 =	13,
> +	RV_REG_A4 =	14,
> +	RV_REG_A5 =	15,
> +	RV_REG_A6 =	16,
> +	RV_REG_A7 =	17,
> +	RV_REG_S2 =	18,	/* Saved registers */
> +	RV_REG_S3 =	19,
> +	RV_REG_S4 =	20,
> +	RV_REG_S5 =	21,
> +	RV_REG_S6 =	22,
> +	RV_REG_S7 =	23,
> +	RV_REG_S8 =	24,
> +	RV_REG_S9 =	25,
> +	RV_REG_S10 =	26,
> +	RV_REG_S11 =	27,
> +	RV_REG_T3 =	28,	/* Temporaries */
> +	RV_REG_T4 =	29,
> +	RV_REG_T5 =	30,
> +	RV_REG_T6 =	31,
> +};
> +
> +struct rv_jit_context {
> +	struct bpf_prog *prog;
> +	u32 *insns; /* RV insns */
> +	int ninsns;
> +	int epilogue_offset;
> +	int *offset; /* BPF to RV */
> +	unsigned long seen_reg_bits;
> +	int stack_size;
> +};
> +
> +struct rv_jit_data {
> +	struct bpf_binary_header *header;
> +	u8 *image;
> +	struct rv_jit_context ctx;
> +};
> +
> +static u8 bpf_to_rv_reg(int bpf_reg, struct rv_jit_context *ctx)
> +{

This one can also be simplified by having a simple mapping as in
other JITs and then mark __set_bit(<reg>) in the small bpf_to_rv_reg()
helper.

> +	switch (bpf_reg) {
> +	/* Return value */
> +	case BPF_REG_0:
> +		__set_bit(RV_REG_A5, &ctx->seen_reg_bits);
> +		return RV_REG_A5;
> +	/* Function arguments */
> +	case BPF_REG_1:
> +		__set_bit(RV_REG_A0, &ctx->seen_reg_bits);
> +		return RV_REG_A0;
> +	case BPF_REG_2:
> +		__set_bit(RV_REG_A1, &ctx->seen_reg_bits);
> +		return RV_REG_A1;
> +	case BPF_REG_3:
> +		__set_bit(RV_REG_A2, &ctx->seen_reg_bits);
> +		return RV_REG_A2;
> +	case BPF_REG_4:
> +		__set_bit(RV_REG_A3, &ctx->seen_reg_bits);
> +		return RV_REG_A3;
> +	case BPF_REG_5:
> +		__set_bit(RV_REG_A4, &ctx->seen_reg_bits);
> +		return RV_REG_A4;
> +	/* Callee saved registers */
> +	case BPF_REG_6:
> +		__set_bit(RV_REG_S1, &ctx->seen_reg_bits);
> +		return RV_REG_S1;
> +	case BPF_REG_7:
> +		__set_bit(RV_REG_S2, &ctx->seen_reg_bits);
> +		return RV_REG_S2;
> +	case BPF_REG_8:
> +		__set_bit(RV_REG_S3, &ctx->seen_reg_bits);
> +		return RV_REG_S3;
> +	case BPF_REG_9:
> +		__set_bit(RV_REG_S4, &ctx->seen_reg_bits);
> +		return RV_REG_S4;
> +	/* Stack read-only frame pointer to access stack */
> +	case BPF_REG_FP:
> +		__set_bit(RV_REG_S5, &ctx->seen_reg_bits);
> +		return RV_REG_S5;
> +	/* Temporary register */
> +	case BPF_REG_AX:
> +		__set_bit(RV_REG_T0, &ctx->seen_reg_bits);
> +		return RV_REG_T0;
> +	/* Tail call counter */
> +	case TAIL_CALL_REG:
> +		__set_bit(RV_REG_S6, &ctx->seen_reg_bits);
> +		return RV_REG_S6;
> +	default:
> +		return 0;
> +	}
> +};
[...]
> +	/* tail call */
> +	case BPF_JMP | BPF_TAIL_CALL:
> +		rd = bpf_to_rv_reg(TAIL_CALL_REG, ctx);
> +		pr_err("bpf-jit: tail call not supported yet!\n");
> +		return -1;

There are two options here, either fixed size prologue where you can
then jump over it in tail call case, or dynamic one which would make
it slower due to reg restore but shrinks image for non-tail calls.

> +	/* function return */
> +	case BPF_JMP | BPF_EXIT:
> +		if (i == ctx->prog->len - 1)
> +			break;
> +
> +		rvoff = epilogue_offset(ctx);
> +		if (!is_21b_int(rvoff)) {
> +			pr_err("bpf-jit: %d offset=%d not supported yet!\n",
> +			       __LINE__, rvoff);
> +			return -1;
> +		}
> +
> +		emit(rv_jal(RV_REG_ZERO, rvoff >> 1), ctx);
> +		break;
> +
> +	/* dst = imm64 */
> +	case BPF_LD | BPF_IMM | BPF_DW:
> +	{
> +		struct bpf_insn insn1 = insn[1];
> +		u64 imm64;
> +
[...]
> +
> +static void build_prologue(struct rv_jit_context *ctx)
> +{
> +	int stack_adjust = 0, store_offset, bpf_stack_adjust;
> +
> +	if (seen_reg(RV_REG_RA, ctx))
> +		stack_adjust += 8;
> +	stack_adjust += 8; /* RV_REG_FP */
> +	if (seen_reg(RV_REG_S1, ctx))
> +		stack_adjust += 8;
> +	if (seen_reg(RV_REG_S2, ctx))
> +		stack_adjust += 8;
> +	if (seen_reg(RV_REG_S3, ctx))
> +		stack_adjust += 8;
> +	if (seen_reg(RV_REG_S4, 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 = round_up(stack_adjust, 16);
> +	bpf_stack_adjust = round_up(ctx->prog->aux->stack_depth, 16);
> +	stack_adjust += bpf_stack_adjust;
> +
> +	store_offset = stack_adjust - 8;
> +
> +	emit(rv_addi(RV_REG_SP, RV_REG_SP, -stack_adjust), ctx);
> +
> +	if (seen_reg(RV_REG_RA, ctx)) {
> +		emit(rv_sd(RV_REG_SP, store_offset, RV_REG_RA), ctx);
> +		store_offset -= 8;
> +	}
> +	emit(rv_sd(RV_REG_SP, store_offset, RV_REG_FP), ctx);
> +	store_offset -= 8;
> +	if (seen_reg(RV_REG_S1, ctx)) {
> +		emit(rv_sd(RV_REG_SP, store_offset, RV_REG_S1), ctx);
> +		store_offset -= 8;
> +	}
> +	if (seen_reg(RV_REG_S2, ctx)) {
> +		emit(rv_sd(RV_REG_SP, store_offset, RV_REG_S2), ctx);
> +		store_offset -= 8;
> +	}
> +	if (seen_reg(RV_REG_S3, ctx)) {
> +		emit(rv_sd(RV_REG_SP, store_offset, RV_REG_S3), ctx);
> +		store_offset -= 8;
> +	}
> +	if (seen_reg(RV_REG_S4, ctx)) {
> +		emit(rv_sd(RV_REG_SP, store_offset, RV_REG_S4), ctx);
> +		store_offset -= 8;
> +	}
> +	if (seen_reg(RV_REG_S5, ctx)) {
> +		emit(rv_sd(RV_REG_SP, store_offset, RV_REG_S5), ctx);
> +		store_offset -= 8;
> +	}
> +	if (seen_reg(RV_REG_S6, ctx)) {
> +		emit(rv_sd(RV_REG_SP, store_offset, RV_REG_S6), ctx);
> +		store_offset -= 8;
> +	}
> +
> +	emit(rv_addi(RV_REG_FP, RV_REG_SP, stack_adjust), ctx);
> +
> +	if (bpf_stack_adjust) {
> +		if (!seen_reg(RV_REG_S5, ctx))
> +			pr_warn("bpf-jit: not seen BPF_REG_FP, stack is %d\n",
> +				bpf_stack_adjust);
> +		emit(rv_addi(RV_REG_S5, RV_REG_SP, bpf_stack_adjust), ctx);
> +	}
> +
> +	ctx->stack_size = stack_adjust;
> +}
> +
> +static void build_epilogue(struct rv_jit_context *ctx)
> +{
> +	int stack_adjust = ctx->stack_size, store_offset = stack_adjust - 8;
> +
> +	if (seen_reg(RV_REG_RA, ctx)) {
> +		emit(rv_ld(RV_REG_RA, store_offset, RV_REG_SP), ctx);
> +		store_offset -= 8;
> +	}
> +	emit(rv_ld(RV_REG_FP, store_offset, RV_REG_SP), ctx);
> +	store_offset -= 8;
> +	if (seen_reg(RV_REG_S1, ctx)) {
> +		emit(rv_ld(RV_REG_S1, store_offset, RV_REG_SP), ctx);
> +		store_offset -= 8;
> +	}
> +	if (seen_reg(RV_REG_S2, ctx)) {
> +		emit(rv_ld(RV_REG_S2, store_offset, RV_REG_SP), ctx);
> +		store_offset -= 8;
> +	}
> +	if (seen_reg(RV_REG_S3, ctx)) {
> +		emit(rv_ld(RV_REG_S3, store_offset, RV_REG_SP), ctx);
> +		store_offset -= 8;
> +	}
> +	if (seen_reg(RV_REG_S4, ctx)) {
> +		emit(rv_ld(RV_REG_S4, store_offset, RV_REG_SP), ctx);
> +		store_offset -= 8;
> +	}
> +	if (seen_reg(RV_REG_S5, ctx)) {
> +		emit(rv_ld(RV_REG_S5, store_offset, RV_REG_SP), ctx);
> +		store_offset -= 8;
> +	}
> +	if (seen_reg(RV_REG_S6, ctx)) {
> +		emit(rv_ld(RV_REG_S6, store_offset, RV_REG_SP), ctx);
> +		store_offset -= 8;
> +	}
> +
> +	emit(rv_addi(RV_REG_SP, RV_REG_SP, stack_adjust), ctx);
> +	/* Set return value. */
> +	emit(rv_addi(RV_REG_A0, RV_REG_A5, 0), ctx);
> +	emit(rv_jalr(RV_REG_ZERO, RV_REG_RA, 0), ctx);
> +}
> +
> +static int build_body(struct rv_jit_context *ctx, bool extra_pass)
> +{
> +	const struct bpf_prog *prog = ctx->prog;
> +	int i;
> +
> +	for (i = 0; i < prog->len; i++) {
> +		const struct bpf_insn *insn = &prog->insnsi[i];
> +		int ret;
> +
> +		ret = emit_insn(insn, ctx, extra_pass);
> +		if (ret > 0) {
> +			i++;
> +			if (ctx->insns == NULL)
> +				ctx->offset[i] = ctx->ninsns;
> +			continue;
> +		}
> +		if (ctx->insns == NULL)
> +			ctx->offset[i] = ctx->ninsns;
> +		if (ret)
> +			return ret;
> +	}
> +	return 0;
> +}
> +
> +static void bpf_fill_ill_insns(void *area, unsigned int size)
> +{
> +	memset(area, 0, size);

Needs update as well?

> +}
> +
> +static void bpf_flush_icache(void *start, void *end)
> +{
> +	flush_icache_range((unsigned long)start, (unsigned long)end);
> +}
> +

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