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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 40133C33CA1 for ; Thu, 9 Jan 2020 12:01:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 15FBB2077B for ; Thu, 9 Jan 2020 12:01:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1578571308; bh=wykY2q8q14HRcxcgRn9igHW61k+IWSYOGFm/09lGxGE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=q0887IVOSssk1yMyihmeQLoNm2nWSF2tCmAMpcr45jDddY0fg0CZkpVT6p0FedDFL MSMJ14EeqKs41OLioyG2dBdvtzXpnL5VaHJ+G6t3aoJ8atydUhyW9LERoMYPKzna2x Yhfy85Q1C0ywB1pvybEEhTUTpl53CwucLVjB3agQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730735AbgAIMBr (ORCPT ); Thu, 9 Jan 2020 07:01:47 -0500 Received: from mail.kernel.org ([198.145.29.99]:50562 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729113AbgAIMBp (ORCPT ); Thu, 9 Jan 2020 07:01:45 -0500 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (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 5177F2075D; Thu, 9 Jan 2020 12:01:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1578571304; bh=wykY2q8q14HRcxcgRn9igHW61k+IWSYOGFm/09lGxGE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=zJiKDhH8Jx3JQmDdSWbVx+GYxcI51ag8Rfy+g+aMsGE5xIQopgNubFHIlCNU3SpiT 3ZiibELRBbcmZeS9pO1hkBHiJciD66To1K1kvN+0cl6sgfK69LyHvTvCTSIhGiWFdY c0z+BLx63ADXa50pFBUBtUvYV7miN+DENeqJp7kU= Date: Thu, 9 Jan 2020 12:01:39 +0000 From: Will Deacon To: Andrew Murray Cc: Catalin Marinas , kvm@vger.kernel.org, Marc Zyngier , linux-kernel@vger.kernel.org, Sudeep Holla , kvmarm , linux-arm-kernel Subject: Re: [PATCH v2 09/18] arm64: KVM: enable conditional save/restore full SPE profiling buffer controls Message-ID: <20200109120139.GA7512@willie-the-truck> 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> <20200108115816.GB15861@willie-the-truck> <745529f7e469b898b74dfc5153e3daf6@kernel.org> <20200108131020.GB16658@willie-the-truck> <20200109112336.GY42593@e119886-lin.cambridge.arm.com> <20200109112504.GZ42593@e119886-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200109112504.GZ42593@e119886-lin.cambridge.arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 09, 2020 at 11:25:04AM +0000, Andrew Murray wrote: > On Thu, Jan 09, 2020 at 11:23:37AM +0000, Andrew Murray wrote: > > On Wed, Jan 08, 2020 at 01:10:21PM +0000, Will Deacon wrote: > > > On Wed, Jan 08, 2020 at 12:36:11PM +0000, Marc Zyngier wrote: > > > > On 2020-01-08 11:58, Will Deacon wrote: > > > > > On Wed, Jan 08, 2020 at 11:17:16AM +0000, Marc Zyngier wrote: > > > > > > On 2020-01-07 15:13, Andrew Murray wrote: > > > > > > > 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. > > > > I hadn't really noticed the kvm_make_request mechanism - however it's now > > clear how this could be implemented. > > > > This approach gives responsibility for which CPUs should be used to userspace > > and if userspace gets it wrong then the KVM_RUN ioctl won't do very much. > > > > > > > > > > > > > > > > Userspace can always check /sys/devices/arm_spe_0/cpumask and work out > > > > > > where to run that particular vcpu. > > > > > > > > > > It's also worth considering systems where there are multiple > > > > > implementations > > > > > of SPE in play. Assuming we don't want to expose this to a guest, then > > > > > the > > > > > right interface here is probably for userspace to pick one SPE > > > > > implementation and expose that to the guest. > > > > If I understand correctly then this implies the following: > > > > - If the host userspace indicates it wants support for SPE in the guest (via > > KVM_SET_DEVICE_ATTR at start of day) - then we should check in vcpu_load that > > the minimum version of SPE is present on the current CPU. 'minimum' because > > we don't know why userspace has selected the given cpumask. > > > > - Userspace can get it wrong, i.e. it can create a CPU mask with CPUs that > > have SPE with differing versions. If it does, and all CPUs have some form of > > SPE then errors may occur in the guest. Perhaps this is OK and userspace > > shouldn't get it wrong? > > Actually this could be guarded against by emulating the ID_AA64DFR0_EL1 such to > cap the version to the minimum SPE version - if absolutely required. The problem is, it's not as simple as checking a version field. Instead, you'd have to look at all of the ID registers for SPE so that you don't end up with funny differences such as minimum sampling interval, or hardware RNG support. Ultimately though, *much* of the trace is going to be describing IMP DEF stuff because it's so micro-architectural, and there's very little you can do to hide that. Will