Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1947160AbdDYMvO (ORCPT <rfc822;w@1wt.eu>);
        Tue, 25 Apr 2017 08:51:14 -0400
Received: from mail-wr0-f174.google.com ([209.85.128.174]:36456 "EHLO
        mail-wr0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1946321AbdDYMvI (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 25 Apr 2017 08:51:08 -0400
Date: Tue, 25 Apr 2017 14:51:03 +0200
From: Daniel Lezcano <daniel.lezcano@linaro.org>
To: Marc Zyngier <marc.zyngier@arm.com>
Cc: tglx@linutronix.de, Mark Rutland <mark.rutland@arm.com>,
        Vineet Gupta <vgupta@synopsys.com>,
        Patrice Chotard <patrice.chotard@st.com>,
        Kukjin Kim <kgene@kernel.org>, Krzysztof Kozlowski <krzk@kernel.org>,
        Javier Martinez Canillas <javier@osg.samsung.com>,
        Christoffer Dall <christoffer.dall@linaro.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-snps-arc@lists.infradead.org, kernel@stlinux.com,
        linux-samsung-soc@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
        kvm@vger.kernel.org
Subject: Re: [PATCH V9 1/3] irq: Allow to pass the IRQF_TIMER flag with
 percpu irq request
Message-ID: <20170425125103.GC16888@mai>
References: <1493042494-14057-1-git-send-email-daniel.lezcano@linaro.org>
 <398f3f3d-c567-0f1f-1a43-9b8d5805d5fd@arm.com>
 <20170424185909.GD2137@mai>
 <92e2a022-93d4-d4f3-78af-c9b5d51bb867@arm.com>
 <20170424195948.GE2137@mai>
 <16042494-2e67-e1a5-b9f6-af57e349d8a7@arm.com>
 <20170425083451.GA16888@mai>
 <f840e9d2-8aac-dc1d-6468-44f7658b55b1@arm.com>
 <20170425094927.GB16888@mai>
 <c98a5ff3-c2ea-5f94-d136-391bd7c224f3@arm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <c98a5ff3-c2ea-5f94-d136-391bd7c224f3@arm.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2264
Lines: 54

On Tue, Apr 25, 2017 at 11:21:21AM +0100, Marc Zyngier wrote:
> On 25/04/17 10:49, Daniel Lezcano wrote:
> > On Tue, Apr 25, 2017 at 10:10:12AM +0100, Marc Zyngier wrote:
> 
> [...]
> 
> >>> +static inline void setup_timings(struct irq_desc *desc, struct irqaction *act)
> >>> +{
> >>> +	/*
> >>> +	 * We don't need the measurement because the idle code already
> >>> +	 * knows the next expiry event.
> >>> +	 */
> >>> +	if (act->flags & __IRQF_TIMER)
> >>> +		return;
> >>
> >> And that's where this is really wrong for the KVM guest timer. As I
> >> said, this timer is under complete control of the guest, and the rest of
> >> the system doesn't know about it. KVM itself will only find out when the
> >> vcpu does a VM exit for a reason or another, and will just save/restore
> >> the state in order to be able to give the timer to another guest.
> >>
> >> The idle code is very much *not* aware of anything concerning that guest
> >> timer.
> > 
> > Just for my own curiosity, if there are two VM (VM1 and VM2). VM1 sets a timer1
> > at <time> and exits, VM2 runs and sets a timer2 at <time+delta>.
> > 
> > The timer1 for VM1 is supposed to expire while VM2 is running. IIUC the virtual
> > timer is under control of VM2 and will expire at <time+delta>.
> > 
> > Is the host wake up with the SW timer and switch in VM1 which in turn restores
> > the timer and jump in the virtual timer irq handler?
> 
> Indeed. The SW timer causes VM1 to wake-up, either on the same CPU
> (preempting VM2) or on another. The timer is then restored with the
> pending virtual interrupt injected, and the guest does what it has to
> with it.

Thanks for clarification.

So there is a virtual timer with real registers / interruption (waking up the
host) for the running VMs and SW timers for non-running VMs.

What is the benefit of having such mechanism instead of real timers injecting
interrupts in the VM without the virtual timer + save/restore? Efficiency in
the running VMs when setting up timers (saving privileges change overhead)?

-- 

 <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog