Re: [PATCH bpf-next v2 1/2] bpf: Introduce bpf_in_interrupt kfunc

[Leon Hwang <leon.hwang@linux.dev>]

On 2/9/25 10:29, Alexei Starovoitov wrote:
> On Mon, Sep 1, 2025 at 8:12 AM Leon Hwang <leon.hwang@linux.dev> wrote:
>>
>>
>> I do a PoC of adding bpf_in_interrupt() to bpf_experimental.h.
> 
> lgtm
> 
>> It works:
>>
>> extern bool CONFIG_PREEMPT_RT __kconfig __weak;
>> #ifdef bpf_target_x86
> 
> what is bpf_target_x86 ?
> 

bpf_target_x86 is a macro defined in bpf_tracing.h:

#if defined(__TARGET_ARCH_x86)
        #define bpf_target_x86
...
#if defined(__x86_64__)
        #define bpf_target_x86
...

>> extern const int __preempt_count __ksym;
>> #endif
>>
>> struct task_struct__preempt_rt {
>>         int softirq_disable_cnt;
>> } __attribute__((preserve_access_index));
>>
>> /* Description
>>  *      Report whether it is in interrupt context. Only works on x86.
> 
> arm64 shouldn't be hard to support either.
> 

No problem.

I'll add support for arm64.

>>  */
>> static inline int bpf_in_interrupt(void)
>> {
>> #ifdef bpf_target_x86
>>         int pcnt;
>>
>>         pcnt = *(int *) bpf_this_cpu_ptr(&__preempt_count);
>>         if (!CONFIG_PREEMPT_RT) {
>>                 return pcnt & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_MASK);
>>         } else {
>>                 struct task_struct__preempt_rt *tsk;
>>
>>                 tsk = (void *) bpf_get_current_task_btf();
>>                 return (pcnt & (NMI_MASK | HARDIRQ_MASK)) |
>>                        (tsk->softirq_disable_cnt | SOFTIRQ_MASK);
>>         }
>> #else
>>         return 0;
>> #endif
>> }
>>
>> However, I only test it for !PREEMPT_RT on x86.
>>
>> I'd like to respin the patchset by moving bpf_in_interrupt() to
>> bpf_experimental.h.
> 
> I think the approach should work for PREEMPT_RT too.
> Test it and send it.

Yes, it works for PREEMPT_RT too.

It is able to get retval of bpf_in_interrupt() by running a simple demo.

However, it's really difficult to add selftest case for PREEMPT_RT, as
I'm not familiar with PREEMPT_RT.

Thanks,
Leon

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