Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752701AbaGaQuJ (ORCPT ); Thu, 31 Jul 2014 12:50:09 -0400 Received: from mail-la0-f52.google.com ([209.85.215.52]:58837 "EHLO mail-la0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751349AbaGaQuG (ORCPT ); Thu, 31 Jul 2014 12:50:06 -0400 Date: Thu, 31 Jul 2014 18:50:06 +0200 From: Christoffer Dall To: Peter Maydell Cc: Alex =?iso-8859-1?Q?Benn=E9e?= , kvm-devel , Catalin Marinas , Will Deacon , open list , Gleb Natapov , Paolo Bonzini , "kvmarm@lists.cs.columbia.edu" , arm-mail-list Subject: Re: [PATCH] arm64: KVM: export current vcpu->pause state via pseudo regs Message-ID: <20140731165006.GL11610@cbox> References: <1404914112-7298-1-git-send-email-alex.bennee@linaro.org> <20140731143538.GI11610@cbox> <87mwbpimgz.fsf@linaro.org> <20140731163805.GK11610@cbox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 31, 2014 at 05:45:28PM +0100, Peter Maydell wrote: > On 31 July 2014 17:38, Christoffer Dall wrote: > >> > If we are not complaining when setting the pause value to false if it > >> > was true before, then we probably also need to wake up the thread in > >> > case this is called from another thread, right? > >> > > >> > or perhaps we should just return an error if you're trying to un-pause a > >> > CPU through this interface, hmmmm. > >> > >> Wouldn't it be an error to mess with any register when the system is not > >> in a quiescent state? I was assuming that the wake state is dealt with > >> when the run loop finally restarts. > >> > > > > The ABI doesn't really define it as an error (the ABI doesn't enforce > > anything right now) so the question is, does it ever make sense to clear > > the pause flag through this ioctl? If not, I think we should just err > > on the side of caution and specify in the docs that this is not > > supported and return an error. > > Consider the case where the reset state of the system is > "CPU 0 running, CPUs 1..N stopped", and we're doing an > incoming migration to a state where all CPUs are running. > In that case we'll be using this ioctl to clear the pause flag, > right? (We'll also obviously need to set the PC and other > register state correctly before resuming the guest.) > Doh, you're right, I somehow had it in my mind that when you send the thread a signal, the pause flag would be cleared, but that goes against the whole idea of a CPU being turned off for KVM. But wouldn't we then have to also wake up the thread when clearing the pause flag? It feels strange that the ioctl can clear the pause flag, but keep the thread on a wake-queue, and then userspace has to send the thread a signal of some sort to wake it up? -Christoffer -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/