From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753910AbbAERqT (ORCPT ); Mon, 5 Jan 2015 12:46:19 -0500 Received: from rhlx01.hs-esslingen.de ([129.143.116.10]:37929 "EHLO rhlx01.hs-esslingen.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751485AbbAERqS (ORCPT ); Mon, 5 Jan 2015 12:46:18 -0500 Date: Mon, 5 Jan 2015 18:46:16 +0100 From: Andreas Mohr To: Purcareata Bogdan Cc: benh@kernel.crashing.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Subject: Re: [RFC] PPC: MPIC: necessary readback after EOI? Message-ID: <20150105174616.GA3159@rhlx01.hs-esslingen.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <54AA9C4D.3000001@freescale.com> X-Priority: none User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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