From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kristina Martsenko Subject: Re: [PATCH v5 2/5] arm64/kvm: preserve host HCR_EL2/MDCR_EL2 value Date: Fri, 15 Feb 2019 15:50:21 +0000 Message-ID: <7ec2f950-7587-5ecd-6caa-c2fd091ad22c@arm.com> References: <1548658727-14271-1-git-send-email-amit.kachhap@arm.com> <1548658727-14271-3-git-send-email-amit.kachhap@arm.com> <8de32a89-4aa1-bffb-78dd-45feddc9216e@arm.com> <1a2534b9-6d3b-caac-ff05-7795b7a2d725@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1a2534b9-6d3b-caac-ff05-7795b7a2d725@arm.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Amit Daniel Kachhap , linux-arm-kernel@lists.infradead.org Cc: Christoffer Dall , Marc Zyngier , Catalin Marinas , Will Deacon , Andrew Jones , Dave Martin , Ramana Radhakrishnan , kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, Mark Rutland List-Id: kvmarm@lists.cs.columbia.edu On 14/02/2019 11:03, Amit Daniel Kachhap wrote: > Hi, > > On 2/13/19 11:04 PM, Kristina Martsenko wrote: >> On 28/01/2019 06:58, Amit Daniel Kachhap wrote: >>> When restoring HCR_EL2 for the host, KVM uses HCR_HOST_VHE_FLAGS, which >>> is a constant value. This works today, as the host HCR_EL2 value is >>> always the same, but this will get in the way of supporting extensions >>> that require HCR_EL2 bits to be set conditionally for the host. >>> >>> To allow such features to work without KVM having to explicitly handle >>> every possible host feature combination, this patch has KVM save/restore >>> the host HCR when switching to/from a guest HCR. The saving of the >>> register is done once during cpu hypervisor initialization state and is >>> just restored after switch from guest. >> >> Why is this patch needed? I couldn't find anything in this series that >> sets HCR_EL2 conditionally for the host. It seems like the kernel still >> always sets it to HCR_HOST_VHE_FLAGS/HCR_HOST_NVHE_FLAGS. > > This patch is not directly related to pointer authentication but just a > helper to optimize save/restore. In this way save may be avoided for > each switch and only restore is done. Patch 3 does sets HCR_EL2 in VHE_RUN. Patch 3 sets the HCR_EL2.{API,APK} bits for the *guest*, not the host. This patch here adds saving/restoring for the *host* HCR_EL2. As far as I can tell, the value of the host HCR_EL2 never changes. Regarding save/restore, currently the kernel never saves the host HCR_EL2, because it always restores HCR_EL2 to HCR_HOST_{,N}VHE_FLAGS (a constant value!) when returning to the host. With this patch, we effectively just save HCR_HOST_{,N}VHE_FLAGS into kvm_host_cpu_state, and restore it from there when returning to the host. Unless we actually change the host HCR_EL2 value to something other than HCR_HOST_{,N}VHE_FLAGS somewhere in this series, this patch is unnecessary. >> >> Looking back at v2 of the userspace pointer auth series, it seems that >> the API/APK bits were set conditionally [1], so this patch would have >> been needed to preserve HCR_EL2. But as of v3 of that series, the bits >> have been set unconditionally through HCR_HOST_NVHE_FLAGS [2]. >> >> Is there something else I've missed? > Now HCR_EL2 is modified during switch time and NHVE doesnt support > ptrauth so [2] doesn't makes sense. In case of NVHE, we do support pointer auth in the *host* userspace, so the patch [2] is necessary. In case of NVHE we do not support pointer auth for KVM *guests*. Thanks, Kristina >> [1] https://lore.kernel.org/linux-arm-kernel/20171127163806.31435-6-mark.rutland@arm.com/ >> [2] https://lore.kernel.org/linux-arm-kernel/20180417183735.56985-5-mark.rutland@arm.com/