linux-kernel - Re: [PATCH] arm64: perf: Fix callchain parse error with kernel tracepoint events

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Date:	Thu, 30 Apr 2015 09:59:49 +0100
From:	Will Deacon <will.deacon@....com>
To:	Hou Pengyang <houpengyang@...wei.com>
Cc:	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"acme@...nel.org" <acme@...nel.org>,
	"wangnan0@...wei.com" <wangnan0@...wei.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Mark Rutland <Mark.Rutland@....com>
Subject: Re: [PATCH] arm64: perf: Fix callchain parse error with kernel
 tracepoint events

On Thu, Apr 30, 2015 at 02:50:05AM +0100, Hou Pengyang wrote:
> On 2015/4/29 18:12, Will Deacon wrote:
> > On Tue, Apr 28, 2015 at 02:20:48PM +0100, Hou Pengyang wrote:
> >> diff --git a/arch/arm64/include/asm/perf_event.h b/arch/arm64/include/asm/perf_event.h
> >> index d26d1d5..16a074f 100644
> >> --- a/arch/arm64/include/asm/perf_event.h
> >> +++ b/arch/arm64/include/asm/perf_event.h
> >> @@ -24,4 +24,20 @@ extern unsigned long perf_misc_flags(struct pt_regs *regs);
> >>   #define perf_misc_flags(regs)	perf_misc_flags(regs)
> >>   #endif
> >>
> >> +#define perf_arch_fetch_caller_regs(regs, __ip) { \
> >> +   unsigned long sp;   \
> >> +   __asm__ ("mov %[sp], sp\n" : [sp] "=r" (sp)); \
> >> +       (regs)->pc = (__ip);    \
> >> +       __asm__ (      \
> >> +               "str %[sp],  %[_arm64_sp]  \n\t"    \
> >> +               "str x29, %[_arm64_fp]  \n\t"    \
> >> +               "mrs %[_arm64_cpsr], spsr_el1 \n\t"     \
> >> +               : [_arm64_sp] "=m" (regs->sp),      \
> >> +                 [_arm64_fp] "=m" (regs->regs[29]),  \
> >> +                 [_arm64_cpsr] "=r" (regs->pstate) \
> >
> > Does this really all need to be in assembly code? Ideally we'd use something
> > like __builtin_stack_pointer and __builtin_frame_pointer. That just leaves
> > the CPSR, but given that it's (a) only used for user_mode_regs tests and (b)
> > this macro is only used by ftrace, then we just set it to a static value
> > indicating that we're at EL1.
> >
> > So I *think* we should be able to write this as three lines of C.
> >
> Hi, will, as you said, we can get fp by __builtin_frame_address() and 
> pstate by setting it to a static value. However, for sp, there isn't a 
> gcc builtin fuction like __builtin_stack_pointer, so assembly code is 
> needed. What's more, if CONFIG_FRAME_POINTER is close, can fp be got by 
> __builtin_frame_address()?

Ah yes, I forgot the history of __builtin_stack_pointer (I think the LLVM
guys proposed it and it was rejected by GCC). Anyway, we can use
current_stack_pointer() instead.

I don't think CONFIG_FRAME_POINTER is relevant here; if you don't have
frame pointers then you won't be able to backtrace. The same issue happens
with your proposed patch.

Will
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