From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Martin Subject: Re: [PATCH 1/2] KVM: arm/arm64: Add save/restore support for firmware workaround state Date: Fri, 15 Feb 2019 17:26:02 +0000 Message-ID: <20190215172558.GO3567@e103592.cambridge.arm.com> References: <20190107120537.184252-1-andre.przywara@arm.com> <20190107120537.184252-2-andre.przywara@arm.com> <20190122151714.GG3578@e103592.cambridge.arm.com> <20190125144657.3db91c91@donnerap.cambridge.arm.com> <20190129213223.GB3567@e103592.cambridge.arm.com> <20190130113900.10089070@donnerap.cambridge.arm.com> <20190215095857.2fd7e0fb@donnerap.cambridge.arm.com> <864l95s2fw.wl-marc.zyngier@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-arm-kernel@lists.infradead.org, Andre Przywara , kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org To: Marc Zyngier Return-path: Content-Disposition: inline In-Reply-To: <864l95s2fw.wl-marc.zyngier@arm.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu List-Id: kvm.vger.kernel.org On Fri, Feb 15, 2019 at 11:42:27AM +0000, Marc Zyngier wrote: > On Fri, 15 Feb 2019 09:58:57 +0000, > Andre Przywara wrote: > > > > On Wed, 30 Jan 2019 11:39:00 +0000 > > Andre Przywara wrote: > > > > Peter, Marc, Christoffer, > > > > can we have an opinion on whether it's useful to introduce some > > common scheme for firmware workaround system registers (parts of > > KVM_REG_ARM_FW_REG(x)), which would allow checking them for > > compatibility between two kernels without specifically knowing about > > them? > > Dave suggested to introduce some kind of signed encoding in the 4 > > LSBs for all those registers (including future ones), where 0 means > > UNKNOWN and greater values are better. So without knowing about the > > particular register, one could judge whether it's safe to migrate. > > I am just not sure how useful this is, given that QEMU seems to ask > > the receiving kernel about any sysreg, and doesn't particularly care > > about the meaning of those registers. And I am not sure we really > > want to introduce some kind of forward looking scheme in the kernel > > here, short of a working crystal ball. I think the kernel policy was > > always to be as strict as possible about those things. > > I honestly don't understand how userspace can decide whether a given > configuration is migratable or not solely based on the value of such a > register. In my experience, the target system has a role to play, and > is the only place where we can find out about whether migration is > actually possible. Both origin and target system need to be taken into account. I don't think that's anything new. > As you said, userspace doesn't interpret the data, nor should it. It > is only on the receiving end that compatibility is assessed and > whether some level of compatibility can be safely ensured. > > So to sum it up, I don't believe in this approach as a general way of > describing the handling or errata. For context, my idea attempted to put KVM, not userspace, in charge of the decision: userspace applies fixed comparison rules determined ahead of time, but KVM supplies the values compared (and hence determines the result). My worry was that otherwise we may end up with a wild-west tangle of arbitrary properties that userspace needs specific knowledge about. We can tolerate a few though. If we accumulate a significant number of errata/vulnerability properties that need to be reported to userspace, this may be worth revisiting. If not, it doesn't matter. Cheers ---Dave 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=-2.5 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham 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 6E1F6C43381 for ; Fri, 15 Feb 2019 17:26:17 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 3E9F621929 for ; Fri, 15 Feb 2019 17:26:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="guBecoDa" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3E9F621929 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=7VQpN3SOfiUtAAi4MIH8FxZTtZ3KeZtme4w1my4pUo4=; b=guBecoDarSHWch k1E300LnZ1TlyW0USiIBQC/WV0PNI5VyTTrLqeEi/NmIQPnIU8ze1y/kG4IgRNoNJAOXHk0rDkCDb NOrS16fsPn0njvNefY0IKIsjUGHbDhqEnsLDXSN6NLtS+DX41n5jhZY25sVNMSFv2n54/xrFPx3De CzbMOksXq3gAoO+BF4e6MXzpJOmuz9mumLRjiQGeT6lOBsTKGt2Wjm3TEvSSz68NOI/cHu26fHA2d b4FWDMBwDJS2GEw3qBI5MFRaGbDmzpt+Z6StXQLqd24Sul9XJdKT69g5wjXxRo4+Xyo7H6hDl7Th7 MeF9GOCU8BUUzmr6JRqA==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1guhFb-0000qu-1c; Fri, 15 Feb 2019 17:26:11 +0000 Received: from foss.arm.com ([217.140.101.70]) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1guhFY-0000qE-34 for linux-arm-kernel@lists.infradead.org; Fri, 15 Feb 2019 17:26:09 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id EAB80EBD; Fri, 15 Feb 2019 09:26:05 -0800 (PST) Received: from e103592.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D94B13F557; Fri, 15 Feb 2019 09:26:04 -0800 (PST) Date: Fri, 15 Feb 2019 17:26:02 +0000 From: Dave Martin To: Marc Zyngier Subject: Re: [PATCH 1/2] KVM: arm/arm64: Add save/restore support for firmware workaround state Message-ID: <20190215172558.GO3567@e103592.cambridge.arm.com> References: <20190107120537.184252-1-andre.przywara@arm.com> <20190107120537.184252-2-andre.przywara@arm.com> <20190122151714.GG3578@e103592.cambridge.arm.com> <20190125144657.3db91c91@donnerap.cambridge.arm.com> <20190129213223.GB3567@e103592.cambridge.arm.com> <20190130113900.10089070@donnerap.cambridge.arm.com> <20190215095857.2fd7e0fb@donnerap.cambridge.arm.com> <864l95s2fw.wl-marc.zyngier@arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <864l95s2fw.wl-marc.zyngier@arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190215_092608_132361_9D0D1247 X-CRM114-Status: GOOD ( 22.36 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arm-kernel@lists.infradead.org, Andre Przywara , kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, Feb 15, 2019 at 11:42:27AM +0000, Marc Zyngier wrote: > On Fri, 15 Feb 2019 09:58:57 +0000, > Andre Przywara wrote: > > > > On Wed, 30 Jan 2019 11:39:00 +0000 > > Andre Przywara wrote: > > > > Peter, Marc, Christoffer, > > > > can we have an opinion on whether it's useful to introduce some > > common scheme for firmware workaround system registers (parts of > > KVM_REG_ARM_FW_REG(x)), which would allow checking them for > > compatibility between two kernels without specifically knowing about > > them? > > Dave suggested to introduce some kind of signed encoding in the 4 > > LSBs for all those registers (including future ones), where 0 means > > UNKNOWN and greater values are better. So without knowing about the > > particular register, one could judge whether it's safe to migrate. > > I am just not sure how useful this is, given that QEMU seems to ask > > the receiving kernel about any sysreg, and doesn't particularly care > > about the meaning of those registers. And I am not sure we really > > want to introduce some kind of forward looking scheme in the kernel > > here, short of a working crystal ball. I think the kernel policy was > > always to be as strict as possible about those things. > > I honestly don't understand how userspace can decide whether a given > configuration is migratable or not solely based on the value of such a > register. In my experience, the target system has a role to play, and > is the only place where we can find out about whether migration is > actually possible. Both origin and target system need to be taken into account. I don't think that's anything new. > As you said, userspace doesn't interpret the data, nor should it. It > is only on the receiving end that compatibility is assessed and > whether some level of compatibility can be safely ensured. > > So to sum it up, I don't believe in this approach as a general way of > describing the handling or errata. For context, my idea attempted to put KVM, not userspace, in charge of the decision: userspace applies fixed comparison rules determined ahead of time, but KVM supplies the values compared (and hence determines the result). My worry was that otherwise we may end up with a wild-west tangle of arbitrary properties that userspace needs specific knowledge about. We can tolerate a few though. If we accumulate a significant number of errata/vulnerability properties that need to be reported to userspace, this may be worth revisiting. If not, it doesn't matter. Cheers ---Dave _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel