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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Nov 23, 2017, at 5:51 PM, Thomas Gleixner tglx@linutronix.de wrote:

> On Thu, 23 Nov 2017, Mathieu Desnoyers wrote:
>> ----- On Nov 22, 2017, at 2:37 PM, Will Deacon will.deacon@arm.com wrote:
>> > On Wed, Nov 22, 2017 at 08:32:19PM +0100, Peter Zijlstra wrote:
>> >>
>> >> 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.
> 
> If we have users of rseq which can do without the other muck, then what's
> the reason not to support it?
> 
> The sysops thing can be sorted out on top and the use cases which need both
> will have to test for both syscalls being available anyway.

I'm currently making sure CONFIG_RSEQ selects both CONFIG_CPU_OPV and
CONFIG_MEMBARRIER, so the user-space fast-paths don't end up with
various ways of doing the fallback/single-stepping/memory barrier handling
depending on whether the kernel support each of those individually.
So first of all, it reduces complexity from a user-space perspective.

Moreover, with a single already needed cpu_id vs cpu_id_start field comparison
in the rseq fast-path, user-space knows that it can rely on having rseq,
cpu_opv, and membarrier. Without this guarantee, user-space would have to
detect individually whether each of those system calls is available, and
test flags on the fast-path, for additional overhead.

Those are my main concerns about pushing an incomplete solution at this
stage.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com