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Subject: Re: [PATCH 1/3] MIPS: kernel: Support extracting off-line stack
 traces from user-space with perf
To:     Peter Zijlstra <peterz@infradead.org>,
        Tiezhu Yang <yangtiezhu@loongson.cn>
Cc:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        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
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>
From:   Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-ID: <0712b131-715a-a83a-bc9e-61405824ff0e@flygoat.com>
Date:   Tue, 5 Jan 2021 11:45:37 +0800
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?? 2021/1/4 ????6:59, Peter Zijlstra д??:
> 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.

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.

> 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.

Thanks.

- Jiaxun