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 4B6ECC433F5 for ; Mon, 17 Jan 2022 13:35:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229826AbiAQNfZ (ORCPT ); Mon, 17 Jan 2022 08:35:25 -0500 Received: from mga02.intel.com ([134.134.136.20]:38483 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229655AbiAQNfY (ORCPT ); Mon, 17 Jan 2022 08:35:24 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1642426524; x=1673962524; h=date:from:to:cc:subject:message-id:references: mime-version:content-transfer-encoding:in-reply-to; bh=3JmoZ0RiGz7FwJ+WYd+MEOkPsuw8YTjwDU2w4Dh7MkI=; b=dXsAeg15WEiujiadsHVqfv7kh7FH0f/GLdkYK6EsZr6xZnnxdbteCumi RhJSHi0dlNCmYbhmAA8vDttR4pqoXU/Q3Qm4t3MH+m+uy4bLgUbZjDxMU Gy1ORlL3YM4FLfBRBPOFrESCxqsPLE+9+fchRFJYueZOVYMElJv4vmuFa PhAnc0D+lkp40OwSNb4fKfQOaACMz2ffCAIawxjFlqG1InNTwvXQ467kx SfmZBG6dLRK+vlkSV2/vxgcgh5lcz/4u99ilaGmt0J6iyqqAKgLfhdx9e 7S3P496vlXP4Z1ZDFipkp88yR+VltMPJh4zXJxF31VWGFL+HN5sfArMks w==; X-IronPort-AV: E=McAfee;i="6200,9189,10229"; a="231981641" X-IronPort-AV: E=Sophos;i="5.88,295,1635231600"; d="scan'208";a="231981641" Received: from fmsmga001.fm.intel.com ([10.253.24.23]) by orsmga101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 17 Jan 2022 05:35:24 -0800 X-IronPort-AV: E=Sophos;i="5.88,295,1635231600"; d="scan'208";a="671559600" Received: from gao-cwp.sh.intel.com (HELO gao-cwp) ([10.239.159.105]) by fmsmga001-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 17 Jan 2022 05:35:20 -0800 Date: Mon, 17 Jan 2022 21:46:10 +0800 From: Chao Gao To: Sean Christopherson Cc: "Tian, Kevin" , "kvm@vger.kernel.org" , "pbonzini@redhat.com" , "tglx@linutronix.de" , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Ingo Molnar , Borislav Petkov , Dave Hansen , "x86@kernel.org" , "H. Peter Anvin" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 5/6] KVM: x86: Remove WARN_ON in kvm_arch_check_processor_compat Message-ID: <20220117134608.GA30004@gao-cwp> References: <20211227081515.2088920-1-chao.gao@intel.com> <20211227081515.2088920-6-chao.gao@intel.com> <20220112110000.GA10249@gao-cwp> <20220117133503.GA27833@gao-cwp> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20220117133503.GA27833@gao-cwp> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Mon, Jan 17, 2022 at 09:35:04PM +0800, Chao Gao wrote: >On Wed, Jan 12, 2022 at 05:35:12PM +0000, Sean Christopherson wrote: >>On Wed, Jan 12, 2022, Chao Gao wrote: >>> On Tue, Jan 11, 2022 at 07:48:39PM +0000, Sean Christopherson wrote: >>> >On Tue, Jan 11, 2022, Tian, Kevin wrote: >>> >> > From: Sean Christopherson >>> >> > Sent: Tuesday, January 11, 2022 7:00 AM >>> >> > >>> >> > On Mon, Dec 27, 2021, Chao Gao wrote: >>> >> > > kvm_arch_check_processor_compat() needn't be called with interrupt >>> >> > > disabled, as it only reads some CRs/MSRs which won't be clobbered >>> >> > > by interrupt handlers or softirq. >>> >> > > >>> >> > > What really needed is disabling preemption. No additional check is >>> >> > > added because if CONFIG_DEBUG_PREEMPT is enabled, smp_processor_id() >>> >> > > (right above the WARN_ON()) can help to detect any violation. >>> >> > >>> >> > Hrm, IIRC, the assertion that IRQs are disabled was more about detecting >>> >> > improper usage with respect to KVM doing hardware enabling than it was >>> >> > about ensuring the current task isn't migrated. E.g. as exhibited by patch >>> >> > 06, extra protections (disabling of hotplug in that case) are needed if >>> >> > this helper is called outside of the core KVM hardware enabling flow since >>> >> > hardware_enable_all() does its thing via SMP function call. >>> >> >>> >> Looks the WARN_ON() was added by you. 😊 >>> > >>> >Yeah, past me owes current me a beer. >>> > >>> >> commit f1cdecf5807b1a91829a2dc4f254bfe6bafd4776 >>> >> Author: Sean Christopherson >>> >> Date: Tue Dec 10 14:44:14 2019 -0800 >>> >> >>> >> KVM: x86: Ensure all logical CPUs have consistent reserved cr4 bits >>> >> >>> >> Check the current CPU's reserved cr4 bits against the mask calculated >>> >> for the boot CPU to ensure consistent behavior across all CPUs. >>> >> >>> >> Signed-off-by: Sean Christopherson >>> >> Signed-off-by: Paolo Bonzini >>> >> >>> >> But it's unclear to me how this WARN_ON() is related to what the commit >>> >> msg tries to explain. >>> > >>> >Ya, the changelog and lack of a comment is awful. >>> > >>> >> When I read this code it's more like a sanity check on the assumption that it >>> >> is currently called in SMP function call which runs the said function with >>> >> interrupt disabled. >>> > >>> >Yes, and as above, that assertion was more about the helper not really being safe >>> >for general usage as opposed to wanting to detect use from preemptible context. >>> >If we end up keeping the WARN_ON, I'll happily write a comment explaining the >>> >point of the assertion. >>> >>> OK. I will do following changes to keep the WARN_ON(): >>> 1. drop this patch >>> 2. disable interrupt before the call site in patch 6. >> >>No, we shouldn't sully other code just to keep this WARN. Again, the point of >>the WARN is/was to highlight that any use outside of the hardware enabling path >>is suspect. That's why I asked if there was a way this code could identify that >>the CPU in question is being hotplugged, i.e. to convey that the helper is safe >>to use only during hardware enabling _or_ hotplug. If that's not feasible, >>replacing the WARN with a scary comment is better than disabling IRQs. > >OK. How about: > > /* > * Compatibility checks are done when loading KVM or in KVM's CPU > * hotplug callback. It ensures all online CPUs are compatible before > * running any vCPUs. For other cases, compatibility checks are > * unnecessary or even problematic. Try to detect improper usages here. > */ > WARN_ON(!irqs_disabled() && !cpu_active(smp_processor_id())); Sorry. It should be: WARN_ON(!irqs_disabled() && cpu_active(smp_processor_id())); > >a CPU is active when it reaches the CPUHP_AP_ACTIVE state (the last state before >CPUHP_ONLINE). So, if a cpu isn't active, it probably is being hotplugged. One >false positive is the CPU is dying, which I guess is fine. > >And to help justify this change, I will merge it into patch 6.