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Date:   Wed, 27 Jan 2021 22:15:06 +0100
From:   Thomas Bogendoerfer <tsbogend@alpha.franken.de>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Tiezhu Yang <yangtiezhu@loongson.cn>,
        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
Subject: Re: [PATCH 1/3] MIPS: kernel: Support extracting off-line stack
 traces from user-space with perf
Message-ID: <20210127211506.GA21163@alpha.franken.de>
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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 ?

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]