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

[Yang Jihong]

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@xxxxxxxxxx>
> > ---
> >   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