Re: [patch 5/8] Immediate Values - x86 Optimization

Mathieu Desnoyers
2007-11-15T04:06:22Z

* Rusty Russell (rusty@rustcorp.com.au) wrote:
> On Wednesday 14 November 2007 05:58:05 Mathieu Desnoyers wrote:
> > x86 optimization of the immediate values which uses a movl with code
> > patching to set/unset the value used to populate the register used as
> > variable source.
>
> For the record, I think the patching code gross overkill.
>
> A stop_machine (or lightweight variant using IPI) would be sufficient and
> vastly simpler. Trying to patch NMI handlers while they're running is
> already crazy.
>

I wouldn't mind if it was limited to the code within do_nmi(), but then
we would have to accept potential GPF if

A - the NMI or MCE code calls any external kernel code (printk,
notify_die, spin_lock/unlock, die_nmi, lapic_wd_event (perfctr code,
calls printk too for debugging)...

B - we try to patch this code at the wrong moment

I could live with that, but I would prefer to have a solid, non flaky
solution. My goal is to help the kernel quality _improve_ rather than
deteriorate. Therefore, if one decides to use the immediate values to
leave dormant spinlock instrumentation in the kernel, I wouldn't want it
to have undesirable side-effects (GPF) when the instrumentation is
being enabled, as rare as it could be.

> I'd keep this version up your sleeve for they day when it's needed.
>

If we choose to go this way, stop_machine would have to do a sync_core()
on every CPU before it reactivates interrupts for this to respect
Intel's errata. It's not just a matter of not executing the code while
it is modified; the issue here is that we must insure that we don't have
an incoherent trace cache. So, as is, stop_machine would not respect
the errata.

Mathieu

> Rusty.