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Date:   Wed, 22 Jul 2020 16:49:47 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        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>
Subject: Re: [PATCH v3 bpf-next 1/2] bpf: separate bpf_get_[stack|stackid] for
 perf events BPF



> On Jul 22, 2020, at 8:40 AM, Peter Zijlstra <peterz@...radead.org> 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

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