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

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Date:	Wed, 3 Oct 2012 15:30:07 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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, Oct 03, 2012 at 03:22:17PM +0200, Peter Zijlstra wrote:
> 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?

right, the init can stay as it was

jirka
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