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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 8D2B5C10F13 for ; Mon, 8 Apr 2019 13:06:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 64583213F2 for ; Mon, 8 Apr 2019 13:06:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726742AbfDHNGB (ORCPT ); Mon, 8 Apr 2019 09:06:01 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:48238 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726349AbfDHNGA (ORCPT ); Mon, 8 Apr 2019 09:06:00 -0400 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 0DB0815BF; Mon, 8 Apr 2019 06:06:00 -0700 (PDT) Received: from [10.162.0.144] (a075553-lin.blr.arm.com [10.162.0.144]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 919783F68F; Mon, 8 Apr 2019 06:05:55 -0700 (PDT) Subject: Re: [PATCH v8 4/9] KVM: arm/arm64: preserve host HCR_EL2 value To: James Morse , 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, Kristina Martsenko , linux-kernel@vger.kernel.org, Mark Rutland , Julien Thierry References: <1554172037-4516-1-git-send-email-amit.kachhap@arm.com> <1554172037-4516-5-git-send-email-amit.kachhap@arm.com> From: Amit Daniel Kachhap Message-ID: <6131d2cb-1062-6331-c2c3-7026081c458a@arm.com> Date: Mon, 8 Apr 2019 18:35:52 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi James, On 4/6/19 4:07 PM, James Morse wrote: > Hi Amit, > > On 02/04/2019 03:27, Amit Daniel Kachhap wrote: >> From: Mark Rutland >> >> 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 >> for 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. >> >> For fetching HCR_EL2 during kvm initialisation, a hyp call is made using >> kvm_call_hyp and is helpful in non-VHE case. >> >> For the hyp TLB maintenance code, __tlb_switch_to_host_vhe() is updated >> to toggle the TGE bit with a RMW sequence, as we already do in >> __tlb_switch_to_guest_vhe(). >> >> The value of hcr_el2 is now stored in struct kvm_cpu_context as both host >> and guest can now use this field in a common way. > > These HCR_EL2 flags have had me confused for quite a while. > I thought this was preserving the value that head.S or cpufeature.c had set, and with > ptrauth we couldn't know what this register should be anymore, the host flags has to vary. > > Kristina's explanation of it[0], clarified things, and with a bit more digging it appears > we always set API/APK, even if the hardware doesn't support the feature (as its harmless). > So we don't need to vary the host flags... API/APK is always set for NVHE host mode. > > My question is, what breaks if this patch isn't merged? (the MDCR change is cleanup we can > do because of this HCR change), is this HCR change just cleanup too? If so, can we merge > ptrauth without either, so we only make the change when its needed? (it will cause some > changes in your patch 7, but I can't see where you depend on the host flags). Yes you are right that this patch does not directly effect pointer authentication functionality but contains several optimizations and cleanups such as, * Removes assigning static flags HCR_HOST_VHE_FLAGS/HCR_HOST_NVHE_FLAGS from switch.c so switching functions now are more generic in nature. * Currently the variation in hcr_el2 flags is across modes (VHE/NVHE). Any future conditional change within those modes in host HCR_EL2 may not effect code changes in switch.c * Save of hcr_el2 done at hyp init time so not expensive switching wise. I am fine on posting it separately also. > > I recall Christoffer wanting to keep the restored DAIF register value on guest-exit static > to avoid extra loads/stores when we know what the value would be. I think the same logic > applies here. Yes the saving of host registers once was suggested by Christoffer. Thanks, Amit D > > You mentioned in the cover letter the series has some history to it! > > > Thanks, > > James > > [0] http://lore.kernel.org/r/7ec2f950-7587-5ecd-6caa-c2fd091ad22c@arm.com >