Re: [PATCH 03/14] x86/entry/64: Fix unwind hints in register clearing code

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Mar 12, 2020 at 02:57:14PM -0500, Josh Poimboeuf wrote:
> On Thu, Mar 12, 2020 at 12:29:29PM -0700, Andy Lutomirski wrote:
> > > On Mar 12, 2020, at 10:31 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > > 
> > > The PUSH_AND_CLEAR_REGS macro zeroes each register immediately after
> > > pushing it.  If an NMI or exception hits after a register is cleared,
> > > but before the UNWIND_HINT_REGS annotation, the ORC unwinder will
> > > wrongly think the previous value of the register was zero.  This can
> > > confuse the unwinding process and cause it to exit early.
> > > 
> > > Because ORC is simpler than DWARF, there are a limited number of unwind
> > > annotation states, so it's not possible to add an individual unwind hint
> > > after each push/clear combination.  Instead, the register clearing
> > > instructions need to be consolidated and moved to after the
> > > UNWIND_HINT_REGS annotation.
> > 
> > I don’t suppose you know how bad t he performance hit is on a non-PTI machine?
> 
> Hm, what does it have to do with PTI?  Should I run a syscall
> microbenchmark?

Mostly that performance with PTI on is abysmal so we don't care about a
few cycles.