linux-kernel - Re: [RFC] PPC: MPIC: necessary readback after EOI?

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Date:	Wed, 07 Jan 2015 15:43:15 +0100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Andreas Mohr <andi@...as.de>
Cc:	Purcareata Bogdan <b43198@...escale.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] PPC: MPIC: necessary readback after EOI?

On Mon, 2015-01-05 at 18:46 +0100, Andreas Mohr wrote:
> Hi,
> 
> > I was curious why the mpic_cpu_read(MPIC_INFO(CPU_WHOAMI)) was there in
> > the first place and if it's still needed. If it's still required, I
> > guess a better approach is to eliminate the call only if the kernel is
> > running on the KVM guest side, where the MPIC is emulated and no longer
> > requires a readback.
> 
> "Why not?"
> 
> A mechanism being "emulated"/"virtual" or not
> may not necessarily be much of a distinction (if at all!).
> The readback might be required
> to properly fulfill all requirements
> of a full state change protocol specification,
> which might easily be the case for both RS(*) and virtual hardware.
> And especially for virtual hardware
> such a "readback" event
> might be an extremely important "end of transaction" marker
> which may often be needed for freeing of temporary resources etc.

In that case it was purely something we added after trial and error to
correct a problem, it's not specified as necessary. Basically it's about
making the store synchronous to the MPIC logic. It's definitely not
necessary on an emulated implementation.

> I'm talking out of my *ss without any MPIC specifics here
> (and especially not why the readback there actually is needed -
> if that doesn't happen to be the case for PCI Posting reasons or some such),
> but it's just intended as food for thought :)
> 
> *) Real Silicon (rather than RL - Real Life)
> 
> HTH,
> 
> Andreas Mohr
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