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 X-Spam-Level: X-Spam-Status: No, score=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0A78CC33C9E for ; Wed, 8 Jan 2020 11:17:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D0C98206F0 for ; Wed, 8 Jan 2020 11:17:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1578482240; bh=BOw+oxULUxi4SWAMJyt3dVvRgIUU2l4r0rjJa2+K9No=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=lgAF1Tuxkq4zWUpOU56CFa2xO3CKRV8b38zShz4q8i785Np17Z9cjfyDwDvjdovvO /j4UkZ898ZpY6G2toY34Ofjwsl0zNkS4b1Gij9ayMb+v2u/kihvrs4CcdxRtyIY5cB RKrtW7NFW6Xe55bF66Tc0dKyjGVPYAKs7RyTAS4Y= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728044AbgAHLRU (ORCPT ); Wed, 8 Jan 2020 06:17:20 -0500 Received: from mail.kernel.org ([198.145.29.99]:54494 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726856AbgAHLRT (ORCPT ); Wed, 8 Jan 2020 06:17:19 -0500 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id A98C520673; Wed, 8 Jan 2020 11:17:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1578482238; bh=BOw+oxULUxi4SWAMJyt3dVvRgIUU2l4r0rjJa2+K9No=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=hQwb5ISuoPH21WTJGlGGLWwDnR2n82LROt+g1wPcEIh2MQFLHHCWO7/dB4/oekUQE FsU91xjNAWVwjTIVCRYH/4aFnnEjLhUtWsU6wmQPdOnfZOrAqbQR3IHIKCN2ipbyJH wMIOLhicp0mRnS6E5gFHDa5Es3C1BAz/pd2U8Jvk= Received: from disco-boy.misterjones.org ([51.254.78.96] helo=www.loen.fr) by disco-boy.misterjones.org with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1ip9Ku-0007XE-No; Wed, 08 Jan 2020 11:17:16 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Wed, 08 Jan 2020 11:17:16 +0000 From: Marc Zyngier To: Andrew Murray Cc: Catalin Marinas , Mark Rutland , will@kernel.org, Sudeep Holla , kvm@vger.kernel.org, kvmarm , linux-arm-kernel , linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 09/18] arm64: KVM: enable conditional save/restore full SPE profiling buffer controls In-Reply-To: <20200107151328.GW42593@e119886-lin.cambridge.arm.com> References: <20191220143025.33853-1-andrew.murray@arm.com> <20191220143025.33853-10-andrew.murray@arm.com> <20191221141325.5a177343@why> <20200107151328.GW42593@e119886-lin.cambridge.arm.com> Message-ID: X-Sender: maz@kernel.org User-Agent: Roundcube Webmail/1.3.8 X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: andrew.murray@arm.com, Catalin.Marinas@arm.com, Mark.Rutland@arm.com, will@kernel.org, Sudeep.Holla@arm.com, kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-01-07 15:13, Andrew Murray wrote: > On Sat, Dec 21, 2019 at 02:13:25PM +0000, Marc Zyngier wrote: >> On Fri, 20 Dec 2019 14:30:16 +0000 >> Andrew Murray wrote: >> >> [somehow managed not to do a reply all, re-sending] >> >> > From: Sudeep Holla >> > >> > Now that we can save/restore the full SPE controls, we can enable it >> > if SPE is setup and ready to use in KVM. It's supported in KVM only if >> > all the CPUs in the system supports SPE. >> > >> > However to support heterogenous systems, we need to move the check if >> > host supports SPE and do a partial save/restore. >> >> No. Let's just not go down that path. For now, KVM on heterogeneous >> systems do not get SPE. > > At present these patches only offer the SPE feature to VCPU's where the > sanitised AA64DFR0 register indicates that all CPUs have this support > (kvm_arm_support_spe_v1) at the time of setting the attribute > (KVM_SET_DEVICE_ATTR). > > Therefore if a new CPU comes online without SPE support, and an > existing VCPU is scheduled onto it, then bad things happen - which I > guess > must have been the intention behind this patch. I guess that was the intent. >> If SPE has been enabled on a guest and a CPU >> comes up without SPE, this CPU should fail to boot (same as exposing a >> feature to userspace). > > I'm unclear as how to prevent this. We can set the FTR_STRICT flag on > the sanitised register - thus tainting the kernel if such a non-SPE CPU > comes online - thought that doesn't prevent KVM from blowing up. Though > I don't believe we can prevent a CPU coming up. At the moment this is > my preferred approach. I'd be OK with this as a stop-gap measure. Do we know of any existing design where only half of the CPUs have SPE? > Looking at the vcpu_load and related code, I don't see a way of saying > 'don't schedule this VCPU on this CPU' or bailing in any way. That would actually be pretty easy to implement. In vcpu_load(), check that that the CPU physical has SPE. If not, raise a request for that vcpu. In the run loop, check for that request and abort if raised, returning to userspace. Userspace can always check /sys/devices/arm_spe_0/cpumask and work out where to run that particular vcpu. > > One solution could be to allow scheduling onto non-SPE VCPUs but wrap > the > SPE save/restore code in a macro (much like kvm_arm_spe_v1_ready) that > reads the non-sanitised feature register. Therefore we don't go bang, > but > we also increase the size of any black-holes in SPE capturing. Though > this > feels like something that will cause grief down the line. > > Is there something else that can be done? How does userspace deal with this? When SPE is only available on half of the CPUs, how does perf work in these conditions? Thanks, M. -- Jazz is not dead. It just smells funny...