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

[Tiezhu Yang]

On 01/28/2021 05:15 AM, Thomas Bogendoerfer wrote:
> 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 ?

If it is possible, I prefer to merge this three patches together
through mips-next tree.

Thanks,
Tiezhu

> Thomas.