From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4055FC433FE for ; Tue, 4 Jan 2022 13:24:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232305AbiADNYK (ORCPT ); Tue, 4 Jan 2022 08:24:10 -0500 Received: from foss.arm.com ([217.140.110.172]:59170 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229884AbiADNYJ (ORCPT ); Tue, 4 Jan 2022 08:24:09 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3304C1FB; Tue, 4 Jan 2022 05:24:09 -0800 (PST) Received: from FVFF77S0Q05N (unknown [10.57.9.1]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 91CD63F774; Tue, 4 Jan 2022 05:24:07 -0800 (PST) Date: Tue, 4 Jan 2022 13:24:02 +0000 From: Mark Rutland To: Frederic Weisbecker Cc: Nicolas Saenz Julienne , maz , Will Deacon , paulmck , linux-arm-kernel , rcu , Thomas Gleixner , kvmarm@lists.cs.columbia.edu, linux-kernel Subject: Re: Possible nohz-full/RCU issue in arm64 KVM Message-ID: References: <20211220161014.GC918551@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211220161014.GC918551@lothringen> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 20, 2021 at 05:10:14PM +0100, Frederic Weisbecker wrote: > On Fri, Dec 17, 2021 at 01:21:39PM +0000, Mark Rutland wrote: > > On Fri, Dec 17, 2021 at 12:51:57PM +0100, Nicolas Saenz Julienne wrote: > > > Hi All, > > > > Hi, > > > > > arm64's guest entry code does the following: > > > > > > int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu) > > > { > > > [...] > > > > > > guest_enter_irqoff(); > > > > > > ret = kvm_call_hyp_ret(__kvm_vcpu_run, vcpu); > > > > > > [...] > > > > > > local_irq_enable(); > > > > > > /* > > > * We do local_irq_enable() before calling guest_exit() so > > > * that if a timer interrupt hits while running the guest we > > > * account that tick as being spent in the guest. We enable > > > * preemption after calling guest_exit() so that if we get > > > * preempted we make sure ticks after that is not counted as > > > * guest time. > > > */ > > > guest_exit(); > > > [...] > > > } > > > > > > > > > On a nohz-full CPU, guest_{enter,exit}() delimit an RCU extended quiescent > > > state (EQS). Any interrupt happening between local_irq_enable() and > > > guest_exit() should disable that EQS. Now, AFAICT all el0 interrupt handlers > > > do the right thing if trggered in this context, but el1's won't. Is it > > > possible to hit an el1 handler (for example __el1_irq()) there? > > > > I think you're right that the EL1 handlers can trigger here and won't exit the > > EQS. > > > > I'm not immediately sure what we *should* do here. What does x86 do for an IRQ > > taken from a guest mode? I couldn't spot any handling of that case, but I'm not > > familiar enough with the x86 exception model to know if I'm looking in the > > right place. > > This is one of the purposes of rcu_irq_enter(). el1 handlers don't call irq_enter()? Due to lockep/tracing/etc ordering, we don't use irq_enter() directly and instead call rcu_irq_enter() and irq_enter_rcu() separately. Critically we only call rcu_irq_enter() for IRQs taken from the idle thread, as this was previously thought to be the only place where we could take an IRQ from an EL1 EQS. See __el1_irq(), __enter_from_kernel_mode(), and __exit_to_kernel_mode() in arch/arm64/kernel/entry-common.c. The latter two are largely analogous to the common irqentry_enter9) and irqentry_exit() helpers in kernel/entry/common.c. We need to either rework the KVM code or that entry code. I'll dig into this a bit more... Thanks, Mark.