linux-kernel - Re: [PATCHv4] perf x86_64: Fix rsp register for system call fast path

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Date:	Wed, 03 Oct 2012 15:22:17 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCHv4] perf x86_64: Fix rsp register for system call fast
 path

On Wed, 2012-10-03 at 15:13 +0200, Jiri Olsa wrote:
> @@ -1190,8 +1191,8 @@ static inline void perf_sample_data_init(struct
> perf_sample_data *data,
>         data->raw  = NULL;
>         data->br_stack = NULL;
>         data->period = period;
> -       data->regs_user.abi = PERF_SAMPLE_REGS_ABI_NONE;
> -       data->regs_user.regs = NULL;
> +       /* Sets abi to PERF_SAMPLE_REGS_ABI_NONE. */
> +       memset(&data->regs_user, 0, sizeof(data->regs_user));
>         data->stack_user_size = 0;
>  } 

Hmm, this will slow down all events, regardless of whether they use any
of that stuff or not. Since the one user actually does something like:

  data->regs_user = *pt_regs;

except it does a memcpy() for some obscure reason, it really doesn't
matter what is in there when uninitialized, right?
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