linux-kernel - Re: [PATCH v2]: perf/x86: store user space frame-pointer value on a sample

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Message-Id: <F6BF3B97-F955-4644-A033-4D20CA7F1922@amacapital.net>
Date:   Tue, 15 May 2018 09:30:05 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2]: perf/x86: store user space frame-pointer value on a sample


> On May 15, 2018, at 1:08 AM, Alexey Budankov <alexey.budankov@...ux.intel.com> wrote:
> 
> 
> Hi Andy,
> 
>> On 09.05.2018 17:54, Peter Zijlstra wrote:
>>> On Tue, May 08, 2018 at 06:21:36PM +0300, Alexey Budankov wrote:
>>> 
>>> Store user space frame-pointer value (BP register) into Perf trace 
>>> on a sample for a process so the value becomes available when 
>>> unwinding call stacks for functions gaining event samples.
>>> 
>>> Signed-off-by: Alexey Budankov <alexey.budankov@...ux.intel.com>
>>> ---
>>> arch/x86/kernel/perf_regs.c | 8 +++++++-
>>> 1 file changed, 7 insertions(+), 1 deletion(-)
>>> 
>>> diff --git a/arch/x86/kernel/perf_regs.c b/arch/x86/kernel/perf_regs.c
>>> index e47b2dbbdef3..8d68658eff7f 100644
>>> --- a/arch/x86/kernel/perf_regs.c
>>> +++ b/arch/x86/kernel/perf_regs.c
>>> @@ -156,7 +156,13 @@ void perf_get_regs_user(struct perf_regs *regs_user,
>> 
>> 
>>>     * Most system calls don't save these registers, don't report them.
>> 
>> ^^^ that worries me and is the reason for the '-1's below. However I
>> think with all the PTI rework this might no longer be true.
>> 
>> The Changelog needs to state that user_regs->bp is in fact valid and
>> ideally point to the commits that makes it so. Also this patch should
>> update that comment.
>> 
>> Cc Andy who keeps better track of all that than me.
> 
> Are there any thoughts so far? Feedback on the matter above is highly appreciated.

Sorry, I missed this. Can you forward the original patch?  I don’t have it.
These days, system calls should save all registers, but I’m not entirely sure I want to promise that they’ll continue to do so forever.