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

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Date:	Mon, 5 Jan 2015 18:46:16 +0100
From:	Andreas Mohr <andi@...as.de>
To:	Purcareata Bogdan <b43198@...escale.com>
Cc:	benh@...nel.crashing.org, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] PPC: MPIC: necessary readback after EOI?

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.

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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