linux-kernel - Re: [PATCH] arm64: perf: Report arm pc registers for compat perf

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Message-ID: <20191112094037.GA32269@lakrids.cambridge.arm.com>
Date:   Tue, 12 Nov 2019 09:40:41 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Seung-Woo Kim <sw0312.kim@...sung.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        will@...nel.org, catalin.marinas@....com, sungguk.na@...sung.com
Subject: Re: [PATCH] arm64: perf: Report arm pc registers for compat perf

On Tue, Nov 12, 2019 at 10:01:41AM +0900, Seung-Woo Kim wrote:
> If perf is built as arm 32-bit, it only reads 15 registers as arm
> 32-bit register map and this breaks dwarf call-chain in compat
> perf because pc register information is not filled.
> Report arm pc registers for 32-bit compat perf.
> 
> Without this, arm 32-bit perf dwarf call-graph shows below
> verbose message:
>   unwind: reg 15, val 0
>   unwind: reg 13, val ffbc6360
>   unwind: no map for 0
> 
> Signed-off-by: Seung-Woo Kim <sw0312.kim@...sung.com>
> ---
>  arch/arm64/kernel/perf_regs.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/arm64/kernel/perf_regs.c b/arch/arm64/kernel/perf_regs.c
> index 0bbac61..d4172e7 100644
> --- a/arch/arm64/kernel/perf_regs.c
> +++ b/arch/arm64/kernel/perf_regs.c
> @@ -24,6 +24,8 @@ u64 perf_reg_value(struct pt_regs *regs, int idx)
>  			return regs->compat_sp;
>  		if ((u32)idx == PERF_REG_ARM64_LR)
>  			return regs->compat_lr;
> +		if ((u32)idx == 15) /* PERF_REG_ARM_PC */
> +			return regs->pc;
>  	}

This doesn't look quite right to me, since perf_regs_value() is
consuming the arm64 index for all other registers (e.g. the LR, in the
patch context).

i.e. this is designed for a native arm64 caller, and the fixup allows it
to view a compat task's registers as-if it were native.

How does this work for a native arm64 perf invocation with a compat
task? I assume it consumers regs->pc, and works as expected?

I suspect we need separate native and compat forms of this function, but
then it's not entirely clear how this should work -- how does this work
for a compat perf analysing a native arm64 binary?

Thanks,
Mark.