linux-kernel - Re: [patch 5/8] Immediate Values

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Date:	Thu, 15 Nov 2007 15:45:18 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@....de>, "H. Peter Anvin" <hpa@...or.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 5/8] Immediate Values - x86 Optimization

On Thursday 15 November 2007 15:06:10 Mathieu Desnoyers wrote:
> * Rusty Russell (rusty@...tcorp.com.au) wrote:
> > 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)...

Sure, but as I pointed out previously, such calls are already best effort.  
You can do very little safely from do_nmi(), and calling printk isn't one of 
them, nor is grabbing a spinlock (well, actually you could as long as it's 
*only* used by NMI handlers.  See any of those?).

> 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.

It's overengineered, since it's less likely than deadlock already.

> > 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.

Yes, I don't think stop_machine is actually what you want anyway, since you 
are happy to run in interrupt context.  An IPI-based scheme is probably 
better, and also has the side effect of iret doing the sync you need, IIUC.

Hope that clarifies,
Rusty.
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