linux-kernel - Re: [PATCH 1/3] MIPS: kernel: Support extracting off-line stack traces from user-space with perf

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Date:   Mon, 4 Jan 2021 11:59:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xuefeng Li <lixuefeng@...ngson.cn>,
        David Daney <david.daney@...ium.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Archer Yan <ayan@...ecomp.com>, x86@...nel.org
Subject: Re: [PATCH 1/3] MIPS: kernel: Support extracting off-line stack
 traces from user-space with perf

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?

The thing x86 does today is look at it's pt_regs state to determine the
actual state.