linux-kernel - Re: [PATCH 1/5] irq_work: Architecture support for remote irq work raise

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Date:	Mon, 12 May 2014 18:29:51 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Kevin Hilman <khilman@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 1/5] irq_work: Architecture support for remote irq work
 raise

On Mon, May 12, 2014 at 01:11:50PM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2014-05-12 at 10:08 +1000, Benjamin Herrenschmidt wrote:
> > On Mon, 2014-05-12 at 01:33 +0200, Frederic Weisbecker wrote:
> > > We are going to extend irq work to support remote queuing.
> > > 
> > > So lets add a cpu argument to arch_irq_work_raise(). The architectures
> > > willing to support that must then provide the backend to raise irq work
> > > IPIs remotely.
> > > 
> > > Initial support is provided for x86 and ARM since they are easily
> > > extended. The other archs that overwrite arch_irq_work_raise() seem
> > > to use local clock interrupts and therefore need deeper rewrite of their
> > > irq work support to implement remote raising.
> > 
> > Well, looks like it's time to turn it into an IPI... It gets a bit more
> > tricky because whether whacking the interrupt controller is safe to
> > do from an NMI is safe or not might depend on that irq controller
> > implementation...
> > 
> > It looks like XICS and MPIC should be safe though, so at least we
> > should be able to cover ppc64, but I'll leave ppc32 alone.
> 
> Correction... that's actually a bit more tricky. We might need an MMIO
> to trigger the IPI. That means potentially having to take a hash miss,
> and we certainly can't do that at NMI time at the moment.
> 
> We *could* hard disable interrupts (which blocks our NMIs since they
> arent't real NMIs, they are just a way to bypass our soft-disable state
> for perf interrupts) for hash_page, but that still makes me somewhat
> nervous.
> 
> Another option would be to add an ioremap flag of some description to
> be able to install bolted hash entries. (It already does so if called
> early enough during boot, so it might actually just work by accident but
> that's an undebuggable horror show waiting to happen if we ever change
> that).
> 
> So needs a bit more thinking on our side.

Yeah, well if we ever end up with native remote irq work, only local raise
will need to be NMI-safe. If that ever helps...
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