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

[Peter Zijlstra <peterz@infradead.org>]

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! :-)