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

On Mon, 2015-01-05 at 18:46 +0100, Andreas Mohr wrote:

Hi,

quoted

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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