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

[Will Deacon]

On Wed, Nov 22, 2017 at 08:32:19PM +0100, Peter Zijlstra wrote:
> 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.

Drive-by "ack" to that. I'd really like a working rseq implementation in
mainline, but I don't much care for another interpreter.

Will