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Subject: Re: [PATCH 1/3] MIPS: kernel: Support extracting off-line stack
 traces from user-space with perf
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>, linux-mips@vger.kernel.org,
        linux-kernel@vger.kernel.org, Xuefeng Li <lixuefeng@loongson.cn>,
        David Daney <david.daney@cavium.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        Archer Yan <ayan@wavecomp.com>
References: <1609246561-5474-1-git-send-email-yangtiezhu@loongson.cn>
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 <20210201104338.GA6484@alpha.franken.de>
 <7c081c6f-bf47-353d-95c0-52e8640dc938@loongson.cn>
 <20210203104009.GE7586@alpha.franken.de>
From:   Tiezhu Yang <yangtiezhu@loongson.cn>
Message-ID: <1ba8402b-77a0-a524-b9f0-55e91841cc20@loongson.cn>
Date:   Wed, 3 Feb 2021 21:12:28 +0800
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On 2/3/21 6:40 PM, Thomas Bogendoerfer wrote:
> On Mon, Feb 01, 2021 at 08:56:06PM +0800, Tiezhu Yang wrote:
>> On 02/01/2021 06:43 PM, Thomas Bogendoerfer wrote:
>>> On Tue, Dec 29, 2020 at 08:55:59PM +0800, Tiezhu Yang wrote:
>>>> +++ b/arch/mips/include/uapi/asm/perf_regs.h
>>>> @@ -0,0 +1,42 @@
>>>> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
>>>> +#ifndef _ASM_MIPS_PERF_REGS_H
>>>> +#define _ASM_MIPS_PERF_REGS_H
>>>> +
>>>> +enum perf_event_mips_regs {
>>>> +	PERF_REG_MIPS_PC,
>>>> +	PERF_REG_MIPS_R1,
>>>> +	PERF_REG_MIPS_R2,
>>>> +	PERF_REG_MIPS_R3,
>>>> +	PERF_REG_MIPS_R4,
>>>> +	PERF_REG_MIPS_R5,
>>>> +	PERF_REG_MIPS_R6,
>>>> +	PERF_REG_MIPS_R7,
>>>> +	PERF_REG_MIPS_R8,
>>>> +	PERF_REG_MIPS_R9,
>>>> +	PERF_REG_MIPS_R10,
>>>> +	PERF_REG_MIPS_R11,
>>>> +	PERF_REG_MIPS_R12,
>>>> +	PERF_REG_MIPS_R13,
>>>> +	PERF_REG_MIPS_R14,
>>>> +	PERF_REG_MIPS_R15,
>>>> +	PERF_REG_MIPS_R16,
>>>> +	PERF_REG_MIPS_R17,
>>>> +	PERF_REG_MIPS_R18,
>>>> +	PERF_REG_MIPS_R19,
>>>> +	PERF_REG_MIPS_R20,
>>>> +	PERF_REG_MIPS_R21,
>>>> +	PERF_REG_MIPS_R22,
>>>> +	PERF_REG_MIPS_R23,
>>>> +	PERF_REG_MIPS_R24,
>>>> +	PERF_REG_MIPS_R25,
>>>> +	/*
>>>> +	 * 26 and 27 are k0 and k1, they are always clobbered thus not
>>>> +	 * stored.
>>>> +	 */
>>> haveing this hole here make all code more complicated. Does it hurt
>>> to have R26 and R27 in the list ?
>> I think there is no effect if have R26 and R27 in the list.
>>
>> In the perf_reg_value(), PERF_REG_MIPS_R{26,27} are default case.
> why make them special ? After all they are real registers and are only
> defined special by current ABIs.


By convention, $26 and $27 are k registers which are reserved for use
by the OS kernel.

Here is an explanation [1]:

"An interrupt handler must save any general - purpose registers that
it is going to use (to be restored at return). But to do so requires
you to modify at least one register first (something like sw $t0, saved_t0
expands to two machine instructions using $at).

This situation is resolved by forbidding user programs from using
two general - purpose registers, $k0 and $k1 (The k stands for kernel,
which an exception handler is part of). The interrupt handler is allowed
to use $k0 and $k1 without having to save or restore their values.
This allows just enough leeway to start saving registers, as well as
making returning from the interrupt handler possible."

[1] 
https://stackoverflow.com/questions/27922315/how-to-use-mips-k0-and-k1-registers


>
>> Should I modify enum perf_event_mips_regs to add R26 and R27,
>> and then send v2?
> yes please.
>
> Thomas.
>