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SubjectRe: [PATCH bpf RESEND 3/4] bpf: Add kernel function call support in 32-bit ARM
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Hello,

On 2022/11/3 19:35, Russell King (Oracle) wrote:
> On Thu, Nov 03, 2022 at 05:21:17PM +0800, Yang Jihong wrote:
>> This patch adds kernel function call support to the 32-bit ARM bpf jit.
>>
>> Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
>> ---
>> arch/arm/net/bpf_jit_32.c | 130 ++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 130 insertions(+)
>>
>> diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
>> index 6a1c9fca5260..51428c82bec6 100644
>> --- a/arch/arm/net/bpf_jit_32.c
>> +++ b/arch/arm/net/bpf_jit_32.c
>> @@ -1337,6 +1337,118 @@ static void build_epilogue(struct jit_ctx *ctx)
>> #endif
>> }
>>
>> +/*
>> + * Input parameters of function in 32-bit ARM architecture:
>> + * The first four word-sized parameters passed to a function will be
>> + * transferred in registers R0-R3. Sub-word sized arguments, for example,
>> + * char, will still use a whole register.
>> + * Arguments larger than a word will be passed in multiple registers.
>> + * If more arguments are passed, the fifth and subsequent words will be passed
>> + * on the stack.
>> + *
>> + * The first for args of a function will be considered for
>> + * putting into the 32bit register R1, R2, R3 and R4.
>> + *
>> + * Two 32bit registers are used to pass a 64bit arg.
>> + *
>> + * For example,
>> + * void foo(u32 a, u32 b, u32 c, u32 d, u32 e):
>> + * u32 a: R0
>> + * u32 b: R1
>> + * u32 c: R2
>> + * u32 d: R3
>> + * u32 e: stack
>> + *
>> + * void foo(u64 a, u32 b, u32 c, u32 d):
>> + * u64 a: R0 (lo32) R1 (hi32)
>> + * u32 b: R2
>> + * u32 c: R3
>> + * u32 d: stack
>> + *
>> + * void foo(u32 a, u64 b, u32 c, u32 d):
>> + * u32 a: R0
>> + * u64 b: R2 (lo32) R3 (hi32)
>> + * u32 c: stack
>> + * u32 d: stack
>
> This code supports both EABI and OABI, but the above is EABI-only.
> Either we need to decide not to support OABI, or we need to add code
> for both. That can probably be done by making:
>
Yes, the OABI situation was not considered here before,
Because I don't have OABI ARM machine, I can't actually verify it,
only EABI is supported.
In the next version, will check whether CONFIG_AEABI is enabled.
>> + for (i = 0; i < fm->nr_args; i++) {
>> + if (fm->arg_size[i] > sizeof(u32)) {
>> + if (arg_regs_idx + 1 < nr_arg_regs) {
>> + /*
>> + * AAPCS states:
>> + * A double-word sized type is passed in two
>> + * consecutive registers (e.g., r0 and r1, or
>> + * r2 and r3). The content of the registers is
>> + * as if the value had been loaded from memory
>> + * representation with a single LDM instruction.
>> + */
>> + if (arg_regs_idx & 1)
>> + arg_regs_idx++;
>
> ... this conditional on IS_ENABLED(CONFIG_AEABI).
>
>> + emit(ARM_LDRD_I(arg_regs[arg_regs_idx], ARM_FP,
>> + EBPF_SCRATCH_TO_ARM_FP(
>> + bpf2a32[BPF_REG_1 + i][1])), ctx);
>
> You probably want to re-use the internals of arm_bpf_get_reg64() to load
> the register.
OK, will re-use arm_bpf_get_reg64 in next version.
>
>> +
>> + arg_regs_idx += 2;
>> + } else {
>> + stack_off = ALIGN(stack_off, STACK_ALIGNMENT);
>> +
>> + emit(ARM_LDRD_I(tmp[1], ARM_FP,
>> + EBPF_SCRATCH_TO_ARM_FP(
>> + bpf2a32[BPF_REG_1 + i][1])), ctx);
>
> Same here.
OK, will re-use arm_bpf_get_reg64 in next version.
>
>> + emit(ARM_STRD_I(tmp[1], ARM_SP, stack_off), ctx);
>
> and the internals of arm_bpf_put_reg64() here. Not all Arm CPUs that
> this code runs on supports ldrd and strd.
>
Yes, the ARM CPUs that do not support LDRD and STRD have not been
considered, will fix in next version.


Thanks,
Yang

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