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

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Date:   Tue, 5 Jan 2021 11:18:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>
Cc:     Tiezhu Yang <yangtiezhu@...ngson.cn>,
        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, 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! :-)