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

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 9/9/2026 9:28 AM, Ravi Bangoria wrote:
>>> 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.

Yeah, this introduces the inconsistency between PERF_SAMPLE_IP and the
pt_regs, but I suppose it should be fine. I didn't look at the details yet,
but all places which call user_mode(regs) should not be impacted in theory.
As long as we ensure the PERF_SAMPLE_IP doesn't leak security information
like kernel address, I suppose it should be fine. 

Thanks.


>
> Thanks,
> Ravi