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Message-ID: <8d184497-fecf-497f-8b4c-bcd4b0a697ce@huaweicloud.com>
Date: Tue, 27 May 2025 16:11:19 +0800
From: Xu Kuohai <xukuohai@...weicloud.com>
To: Alexis Lothoré <alexis.lothore@...tlin.com>,
Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
John Fastabend <john.fastabend@...il.com>,
Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
<martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>,
Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Puranjay Mohan <puranjay@...nel.org>,
Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
Maxime Coquelin <mcoquelin.stm32@...il.com>,
Alexandre Torgue <alexandre.torgue@...s.st.com>,
Florent Revest <revest@...omium.org>
Cc: Bastien Curutchet <bastien.curutchet@...tlin.com>,
ebpf@...uxfoundation.org, Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linux-kselftest@...r.kernel.org,
linux-stm32@...md-mailman.stormreply.com, Xu Kuohai <xukuohai@...wei.com>
Subject: Re: [PATCH bpf-next v2 1/2] bpf, arm64: Support up to 12 function
arguments
On 5/22/2025 6:14 PM, Alexis Lothoré wrote:
[...]
> -static void save_args(struct jit_ctx *ctx, int args_off, int nregs)
> +struct arg_aux {
> + /* how many args are passed through registers, the rest of the args are
> + * passed through stack
> + */
> + int args_in_regs;
> + /* how many registers are used to pass arguments */
> + int regs_for_args;
> + /* how much stack is used for additional args passed to bpf program
> + * that did not fit in original function registers
> + **/
nit: "**/" should be "*/"
> + int bstack_for_args;
> + /* home much stack is used for additional args passed to the
> + * original function when called from trampoline (this one needs
> + * arguments to be properly aligned)
> + */
> + int ostack_for_args;
> +};
> +
> +static int calc_arg_aux(const struct btf_func_model *m,
> + struct arg_aux *a)
> {
> - int i;
> + int stack_slots, nregs, slots, i;
> +
> + /* verifier ensures m->nr_args <= MAX_BPF_FUNC_ARGS */
> + for (i = 0, nregs = 0; i < m->nr_args; i++) {
> + slots = (m->arg_size[i] + 7) / 8;
> + if (nregs + slots <= 8) /* passed through register ? */
> + nregs += slots;
> + else
> + break;
> + }
> +
> + a->args_in_regs = i;
> + a->regs_for_args = nregs;
> + a->ostack_for_args = 0;
> +
> + /* the rest arguments are passed through stack */
> + for (a->ostack_for_args = 0, a->bstack_for_args = 0;
> + i < m->nr_args; i++) {
a->ostack_for_args is initialized twice.
move all initializations before the loop?
> + /* We can not know for sure about exact alignment needs for
> + * struct passed on stack, so deny those
> + */
> + if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG)
> + return -EOPNOTSUPP;
leave the error code as is, namely, return -ENOTSUPP?
> + stack_slots = (m->arg_size[i] + 7) / 8;
> + /* AAPCS 64 C.14: arguments passed on stack must be aligned to
> + * max(8, arg_natural_alignment)
> + */
> + a->bstack_for_args += stack_slots * 8;
> + a->ostack_for_args = round_up(a->ostack_for_args + stack_slots * 8, 8);
since a->ostack_for_args starts from 0 and is always incremented
by multiples of 8, round_up() to 8 is not needed.
> + }
>
> - for (i = 0; i < nregs; i++) {
> - emit(A64_STR64I(i, A64_SP, args_off), ctx);
> - args_off += 8;
> + return 0;
> +}
> +
> +static void clear_garbage(struct jit_ctx *ctx, int reg, int effective_bytes)
> +{
> + if (effective_bytes) {
> + int garbage_bits = 64 - 8 * effective_bytes;
> +#ifdef CONFIG_CPU_BIG_ENDIAN
> + /* garbage bits are at the right end */
> + emit(A64_LSR(1, reg, reg, garbage_bits), ctx);
> + emit(A64_LSL(1, reg, reg, garbage_bits), ctx);
> +#else
> + /* garbage bits are at the left end */
> + emit(A64_LSL(1, reg, reg, garbage_bits), ctx);
> + emit(A64_LSR(1, reg, reg, garbage_bits), ctx);
> +#endif
> }
> }
>
> -static void restore_args(struct jit_ctx *ctx, int args_off, int nregs)
> +static void save_args(struct jit_ctx *ctx, int bargs_off, int oargs_off,
> + const struct btf_func_model *m,
> + const struct arg_aux *a,
> + bool for_call_origin)
> {
> int i;
> + int reg;
> + int doff;
> + int soff;
> + int slots;
> + u8 tmp = bpf2a64[TMP_REG_1];
> +
> + /* store arguments to the stack for the bpf program, or restore
> + * arguments from stack for the original function
> + */
> + for (reg = 0; reg < a->regs_for_args; reg++) {
> + emit(for_call_origin ?
> + A64_LDR64I(reg, A64_SP, bargs_off) :
> + A64_STR64I(reg, A64_SP, bargs_off),
> + ctx);
> + bargs_off += 8;
> + }
> +
> + soff = 32; /* on stack arguments start from FP + 32 */
> + doff = (for_call_origin ? oargs_off : bargs_off);
> +
> + /* save on stack arguments */
> + for (i = a->args_in_regs; i < m->nr_args; i++) {
> + slots = (m->arg_size[i] + 7) / 8;
> + /* AAPCS C.14: additional arguments on stack must be
> + * aligned on max(8, arg_natural_alignment)
> + */
> + soff = round_up(soff, 8);
> + if (for_call_origin)
> + doff = round_up(doff, 8);
since both soff and doff start from multiples of 8 and are
incremented by 8 each time, the two round_up()s are also
not needed.
> + /* verifier ensures arg_size <= 16, so slots equals 1 or 2 */
> + while (slots-- > 0) {
> + emit(A64_LDR64I(tmp, A64_FP, soff), ctx);
> + /* if there is unused space in the last slot, clear
> + * the garbage contained in the space.
> + */
> + if (slots == 0 && !for_call_origin)
> + clear_garbage(ctx, tmp, m->arg_size[i] % 8);
> + emit(A64_STR64I(tmp, A64_SP, doff), ctx);
> + soff += 8;
> + doff += 8;
> + }
> + }
> +}
[...]
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