Re: [RFC PATCH for 4.15 v12 00/22] Restartable sequences and CPU op vector

From: Peter Zijlstra
Date: Wed Nov 22 2017 - 14:33:08 EST

On Tue, Nov 21, 2017 at 10:05:08PM +0000, Mathieu Desnoyers wrote:
> Other than that, I have not received any concrete alternative proposal to
> properly handle single-stepping.

That's not entirely true; amluto did have an alternative in Prague: do
full machine level instruction emulation till the end of the rseq when
it gets 'preempted too often'.

Yes, implementing that will be an absolute royal pain. But it does
remove the whole duplicate/dual program asm/bytecode thing and avoids
the syscall entirely.

And we don't need to do a full x86_64/arch-of-choice emulator for this
either; just as cpu_opv is fairly limited too. We can do a subset that
allows dealing with the known sequences and go from there -- it can
always fall back to not emulating and reverting to the pure rseq with
debug/fwd progress 'issues'.

So what exactly is the problem of leaving out the whole cpu_opv thing
for now? Pure rseq is usable -- albeit a bit cumbersome without
additional debugger support.