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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>,
        Peter Zijlstra <peterz@infradead.org>
References: <1609246561-5474-1-git-send-email-yangtiezhu@loongson.cn>
 <1609246561-5474-2-git-send-email-yangtiezhu@loongson.cn>
 <20210104105904.GK3021@hirez.programming.kicks-ass.net>
 <0712b131-715a-a83a-bc9e-61405824ff0e@flygoat.com>
 <20210105101806.GG3040@hirez.programming.kicks-ass.net>
 <20210127211506.GA21163@alpha.franken.de>
Cc:     Jiaxun Yang <jiaxun.yang@flygoat.com>,
        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>, x86@kernel.org
From:   Tiezhu Yang <yangtiezhu@loongson.cn>
Message-ID: <d2035b91-bcb9-bde4-8009-d81d177b2317@loongson.cn>
Date:   Fri, 29 Jan 2021 10:48:52 +0800
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On 01/28/2021 05:15 AM, Thomas Bogendoerfer wrote:
> On Tue, Jan 05, 2021 at 11:18:06AM +0100, Peter Zijlstra wrote:
>> On Tue, Jan 05, 2021 at 11:45:37AM +0800, Jiaxun Yang wrote:
>>> 在 2021/1/4 下午6:59, Peter Zijlstra 写道:
>>>> On Tue, Dec 29, 2020 at 08:55:59PM +0800, Tiezhu Yang wrote:
>>>>> +u64 perf_reg_abi(struct task_struct *tsk)
>>>>> +{
>>>>> +	if (test_tsk_thread_flag(tsk, TIF_32BIT_REGS))
>>>>> +		return PERF_SAMPLE_REGS_ABI_32;
>>>>> +	else
>>>>> +		return PERF_SAMPLE_REGS_ABI_64;
>>>>> +}
>>>> So we recently changed this on x86 to not rely on TIF flags. IIRC the
>>>> problem is that on x86 you can change the mode of a task without the
>>>> kernel being aware of it. Is something like that possible on MIPS as
>>>> well?
>>> Hi all,
>>>
>>> In MIPS world it's impossible to raise a thread to 64bit without kernel
>>> aware.
>>> Without STATUS.UX set it will trigger reserved instruction exception when
>>> trying
>>> to run 64bit instructions.
>> The other way around is the case on x86, a 64bit program can create and
>> execute 32bit code sections without the kernel being aware. But if
>> clearing STATUS.UX has the same issue as setting it, that should not be
>> a problem for you.
>>
>>> However it may be possible to run with 32bit ABI without
>>> TIF_32BIT_REGS if user program didn't get ELF ABI right. I think
>>> that's out of our current consideration.
>> Fair enough.
>>
>>>> The thing x86 does today is look at it's pt_regs state to determine the
>>>> actual state.
>>> It is possible to look at pt_regs Status.UX bit on MIPS. But it seems
>>> unnecessary
>>> as user can't change it.
>> Ok, good. Then no objection, proceed! :-)
> this patch aims more to mips-next, while patch 2 and 3 are targeting
> tools/perf. Should I take them into mips-next, too ?

If it is possible, I prefer to merge this three patches together
through mips-next tree.

Thanks,
Tiezhu

>
> Thomas.
>