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

From: Mathieu Desnoyers
Date: Thu Nov 23 2017 - 16:14:17 EST

----- On Nov 22, 2017, at 2:37 PM, Will Deacon will.deacon@xxxxxxx wrote:

> 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.

Considering the arm 64 use-case of reading PMU counters from user-space
using rseq to prevent migration, I understand that you're lucky enough to
already have a system call at your disposal that can perform the slow-path
in case of single-stepping.

So yes, your particular case is already covered, but unfortunately that's
not the same situation for other use-cases that have been expressed.



> Will

Mathieu Desnoyers
EfficiOS Inc.