Re: [PATCH v2 bpf-next 1/3] bpf: Introduce bpf_timer

[Yonghong Song <yhs@fb.com>]

On 6/14/21 8:33 PM, Alexei Starovoitov wrote:
> On Fri, Jun 11, 2021 at 3:12 PM Yonghong Song <yhs@fb.com> wrote:
>>> +struct bpf_hrtimer {
>>> +     struct hrtimer timer;
>>> +     struct bpf_map *map;
>>> +     struct bpf_prog *prog;
>>> +     void *callback_fn;
>>> +     void *value;
>>> +};
>>> +
>>> +/* the actual struct hidden inside uapi struct bpf_timer */
>>> +struct bpf_timer_kern {
>>> +     struct bpf_hrtimer *timer;
>>> +     struct bpf_spin_lock lock;
>>> +};
>>
>> Looks like in 32bit system, sizeof(struct bpf_timer_kern) is 64
>> and sizeof(struct bpf_timer) is 128.
>>
>> struct bpf_spin_lock {
>>           __u32   val;
>> };
>>
>> struct bpf_timer {
>>          __u64 :64;
>>          __u64 :64;
>> };
>>
>> Checking the code, we may not have issues as structure
>> "bpf_timer" is only used to reserve spaces and
>> map copy value routine handles that properly.
>>
>> Maybe we can still make it consistent with
>> two fields in bpf_timer_kern mapping to
>> two fields in bpf_timer?
>>
>> struct bpf_timer_kern {
>>          __bpf_md_ptr(struct bpf_hrtimer *, timer);
>>          struct bpf_spin_lock lock;
>> };
> 
> Such alignment of fields is not necessary,
> since the fields are not accessible directly from bpf prog.
> struct bpf_timer_kern needs to fit into struct bpf_timer and
> alignof these two structs needs to be the same.
> That's all. I'll add build_bug_on to make sure.

Sounds good to me. Thanks!