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 29CD7C433F5 for ; Mon, 20 Dec 2021 16:10:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237416AbhLTQKV (ORCPT ); Mon, 20 Dec 2021 11:10:21 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53722 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234155AbhLTQKU (ORCPT ); Mon, 20 Dec 2021 11:10:20 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2A3D2C061574; Mon, 20 Dec 2021 08:10:20 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id E25F4B80F4B; Mon, 20 Dec 2021 16:10:18 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0C5B2C36AE2; Mon, 20 Dec 2021 16:10:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1640016617; bh=7PXWoZ2KL+Ig9fI0vXCHFwz904irEuyU5Cq7Elp6Brs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=snS/I7FVuziSrqgP6ZfsuWLXBqwO3TFWdwCd4KlUP6kHrZWdPjZAlak431ccSc7f1 0xnqlXiaU+m0GPCfzw/4KCwQ459s/JxMMzdjDxIrMFLS5KEMefBAOKnyoZPgGN+57V yZDDQP23LRiRQ/o/aZyzqYRdFaGRSf5VdUan7u8zkjnPHO8Cm1f0I1QKtnbzSOGUxu NC67Go7+vGw+kXMpWvVJ8/SuItVhWPC3+ozfgBw1dgP2EPBocMzWN2YmpUspl4aOBt z35OcWYjFQT47Rm0ZytD8ly2w/1ym3LoU/4GGMFkBYnX4YLXv22meHX9G/bmZIunHh LV4S87CL0Kfvg== Date: Mon, 20 Dec 2021 17:10:14 +0100 From: Frederic Weisbecker To: Mark Rutland 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: <20211220161014.GC918551@lothringen> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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()? Thanks.