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 Return-path: Received: from localhost (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 0B4DD49E3E for ; Fri, 15 Feb 2019 12:26:08 -0500 (EST) Received: from mm01.cs.columbia.edu ([127.0.0.1]) by localhost (mm01.cs.columbia.edu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id BJDUl1VxKbuz for ; Fri, 15 Feb 2019 12:26:06 -0500 (EST) Received: from foss.arm.com (foss.arm.com [217.140.101.70]) by mm01.cs.columbia.edu (Postfix) with ESMTP id AB9D340493 for ; Fri, 15 Feb 2019 12:26:06 -0500 (EST) 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 To: Marc Zyngier Cc: linux-arm-kernel@lists.infradead.org, Andre Przywara , kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org List-Id: kvmarm@lists.cs.columbia.edu 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