Re: [PATCH 2/2] perf/x86: Disable precise sampling for PERF_SAMPLE_STACK_USER

[Ravi Bangoria <ravi.bangoria@amd.com>]

>> Having PERF_SAMPLE_IP be precise and the user registers from the
>> interrupt I believe works for AMD IBS and ARM SPE, but for Intel PEBS
>> there is the ability to use the PEBS register samples for other
>> non-redundant registers. In the x86 driver could we disable PEBS
>> sampling for these registers when doing user stack sampling, so that
>> the sampled user registers match the stack sample? We can keep the
>> PERF_SAMPLE_IP precise, and make all the registers precise when there
>> is no stack sampling.
> 
> That sounds the best way to fix this issue by decoupling PERF_SAMPLE_IP
> with PERF_REG_X86_IP. Then we can keep the precise SAMPLE_IP and the PMI
> context user register snapshot simultaneously.

With this, PERF_SAMPLE_IP and PERF_REG_X86_IP might be from different
privileges. So, any perf code that depends on user_mode(regs) (e.g.
perf_exclude_event(), _REGS_USER, _REGS_INTR, header->misc, etc.) also
needs to be inspected/modified accordingly.

Thanks,
Ravi