Date: Thu, 19 Nov 2015 12:23:15 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: "Wangnan (F)" <wangnan0@huawei.com>, Jiri Olsa <jolsa@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        David Ahern <dsahern@gmail.com>, Milian Wolff <milian.wolff@kdab.com>,
        linux-kernel@vger.kernel.org, pi3orama <pi3orama@163.com>,
        lizefan 00213767 <lizefan@huawei.com>
Subject: Re: [BUG REPORT] perf tools: x86_64: Broken calllchain when sampling
 taken at 'callq' instruction
Message-ID: <20151119112315.GL3816@twins.programming.kicks-ass.net>
References: <564C26C4.2040603@huawei.com>
 <564C3011.8090002@huawei.com>
 <20151118082033.GA24726@gmail.com>
 <564C3A0E.3030502@huawei.com>
 <564C3BAA.4040806@huawei.com>
 <20151119063709.GA14852@gmail.com>
 <564D6FF9.3030105@huawei.com>
 <20151119102300.GA2830@gmail.com>
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On Thu, Nov 19, 2015 at 11:23:00AM +0100, Ingo Molnar wrote:
> PEBS is an asynchronous hardware tracing mechanism, when batched PEBS is used it 
> might not even result in any interruption of execution. The 'pt_regs' does not 
> necessarily correspond to an interrupted, restartable context - we take the RIP 
> from the PEBS machinery and also use LBR and disassembly to determine the previous 
> instruction, before reporting it to user-space.

Note that modern PEBS hardware (hsw+) does the rollback in hardware.
Prior to that we indeed to it manually using the LBR.

As to pt_regs, we construct a franken pt_regs based on the actual PEBS
buffer overflow PMI and bits from the PEBS record (which also includes
some register state). See
arch/x86/kernel/cpu/perf_event_intel_ds.c:setup_pebs_sample_data().

We always copy the flags, ip, bp and sp from the PEBS record into the
interrupt pt_regs.

And note that the PEBS record is constructed at instruction retirement,
so it shows the state _after_ the instruction, with exception of the
(hsw+) real_ip field.

So the unwinder will have to be taught that if the IP points at a stack
altering instruction (call, push, etc.) it will have to 'undo' the
effects on the actual stack (I appreciate this might be 'interesting'
for things like: pop, ret, etc.).

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