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Date:   Tue, 21 Jul 2020 22:51:52 +0000
From:   Song Liu <songliubraving@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "kpsingh@...omium.org" <kpsingh@...omium.org>,
        "brouer@...hat.com" <brouer@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>
Subject: Re: [PATCH v3 bpf-next 1/2] bpf: separate bpf_get_[stack|stackid] for
 perf events BPF



> On Jul 21, 2020, at 3:43 PM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> On Tue, Jul 21, 2020 at 3:40 PM Song Liu <songliubraving@...com> wrote:
>> 
>> We only need to block precise_ip >= 2. precise_ip == 1 is OK.
> 
> Are you sure?
> intel_pmu_hw_config() has:
> if (event->attr.precise_ip) {
>    if (event->attr.sample_type & PERF_SAMPLE_CALLCHAIN)
>            event->attr.sample_type |= __PERF_SAMPLE_CALLCHAIN_EARLY;
> }

The bit that breaks the unwinder was in setup_pebs_fixed_sample_data():

                if (x86_pmu.intel_cap.pebs_format >= 2) {
                        set_linear_ip(regs, pebs->real_ip);
                        regs->flags |= PERF_EFLAGS_EXACT;
                } 

"real_ip" causes the issue. 

But on a second thought, it is probably better also blocks precise_ip == 1, 
to match the logic in intel_pmu_hw_config(). 

Thanks,
Song

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