Re: [PATCH v3 bpf-next 1/2] bpf: separate bpf_get_[stack|stackid] for perf events BPF

[Song Liu]

> On Jul 22, 2020, at 8:40 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> On Tue, Jul 21, 2020 at 10:40:19PM +0000, Song Liu wrote:
> 
>> We only need to block precise_ip >= 2. precise_ip == 1 is OK. 
> 
> Uuuh, how? Anything PEBS would have the same problem. Sure, precise_ip
> == 1 will not correct the IP, but the stack will not match regardless.
> 
> You need IP,SP(,BP) to be a consistent set _AND_ have it match the
> current stack, PEBS simply cannot do that, because the regs get recorded
> (much) earlier than the PMI and the stack can have changed in the
> meantime.
> 

By "OK", I meant unwinder will not report error (in my tests). For 
accurate stack, we should do the same for precise_ip == 1. 

Thanks,
Song