linux-kernel - Re: [PATCH 1/4] x86/events: down with test_thread_flag(TIF

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Date:	Thu, 14 Apr 2016 11:32:07 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Dmitry Safonov <dsafonov@...tuozzo.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Robert Richter <rric@...nel.org>, oprofile-list@...ts.sf.net,
	Dmitry Safonov <0x7f454c46@...il.com>
Subject: Re: [PATCH 1/4] x86/events: down with test_thread_flag(TIF_IA32)

On Thu, Apr 14, 2016 at 11:10 AM, Dmitry Safonov <dsafonov@...tuozzo.com> wrote:
> We can use user_64bit_mode(regs) here instead of thread flag
> because we have full register frame.
>
> Signed-off-by: Dmitry Safonov <dsafonov@...tuozzo.com>
> ---
>  arch/x86/events/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 041e442a3e28..91d101a9a6e9 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -2269,7 +2269,7 @@ perf_callchain_user32(struct pt_regs *regs, struct perf_callchain_entry *entry)
>         struct stack_frame_ia32 frame;
>         const void __user *fp;
>
> -       if (!test_thread_flag(TIF_IA32))
> +       if (user_64bit_mode(regs))
>                 return 0;

Peter, I got lost in the code that calls this.  Are regs coming from
the overflow interrupt's regs, current_pt_regs(), or
perf_get_regs_user?

If it's the perf_get_regs_user, then this should be okay, but passing
in the ABI field directly would be even nicer.  If they're coming from
the overflow interrupt's regs or current_pt_regs(), could we change
that?

It might also be nice to make sure that we call perf_get_regs_user
exactly once per overflow interrupt -- i.e. we could push it into the
main code rather than the regs sampling code.

>
>         cs_base = get_segment_base(regs->cs);
> --
> 2.8.0
>



-- 
Andy Lutomirski
AMA Capital Management, LLC