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:

quoted

On Tue, Dec 29, 2020 at 08:55:59PM +0800, Tiezhu Yang wrote:

quoted

+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.

quoted

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! :-)