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=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,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 BF875C282DA for ; Wed, 17 Apr 2019 15:22:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 98D6820821 for ; Wed, 17 Apr 2019 15:22:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732649AbfDQPWr (ORCPT ); Wed, 17 Apr 2019 11:22:47 -0400 Received: from foss.arm.com ([217.140.101.70]:46590 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732110AbfDQPWr (ORCPT ); Wed, 17 Apr 2019 11:22:47 -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 6CF43A78; Wed, 17 Apr 2019 08:22:46 -0700 (PDT) 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 C31D93F557; Wed, 17 Apr 2019 08:22:44 -0700 (PDT) Date: Wed, 17 Apr 2019 16:22:42 +0100 From: Dave Martin To: Amit Daniel Kachhap Cc: Marc Zyngier , Catalin Marinas , Will Deacon , linux-kernel@vger.kernel.org, Kristina Martsenko , Ramana Radhakrishnan , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v9 4/5] KVM: arm64: Add capability to advertise ptrauth for guest Message-ID: <20190417152242.GC3567@e103592.cambridge.arm.com> References: <1555039236-10608-1-git-send-email-amit.kachhap@arm.com> <1555039236-10608-5-git-send-email-amit.kachhap@arm.com> <20190416163212.GX3567@e103592.cambridge.arm.com> <0070b1c2-07d6-7472-1bbc-c252710f6ca3@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0070b1c2-07d6-7472-1bbc-c252710f6ca3@arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 17, 2019 at 03:09:02PM +0530, Amit Daniel Kachhap wrote: > Hi, > > On 4/16/19 10:02 PM, Dave Martin wrote: > >On Fri, Apr 12, 2019 at 08:50:35AM +0530, Amit Daniel Kachhap wrote: > >>This patch advertises the capability of two cpu feature called address > >>pointer authentication and generic pointer authentication. These > >>capabilities depend upon system support for pointer authentication and > >>VHE mode. > >> > >>The current arm64 KVM partially implements pointer authentication and > >>support of address/generic authentication are tied together. However, > >>separate ABI requirements for both of them is added so that any future > >>isolated implementation will not require any ABI changes. > >> > >>Signed-off-by: Amit Daniel Kachhap > >>Cc: Mark Rutland > >>Cc: Marc Zyngier > >>Cc: Christoffer Dall > >>Cc: kvmarm@lists.cs.columbia.edu > >>--- > >>Changes since v8: > >>* Keep the capability check same for the 2 vcpu ptrauth features. [Dave Martin] > >> > >> Documentation/virtual/kvm/api.txt | 2 ++ > >> arch/arm64/kvm/reset.c | 5 +++++ > >> include/uapi/linux/kvm.h | 2 ++ > >> 3 files changed, 9 insertions(+) > >> > >>diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt > >>index 9d202f4..56021d0 100644 > >>--- a/Documentation/virtual/kvm/api.txt > >>+++ b/Documentation/virtual/kvm/api.txt > >>@@ -2756,9 +2756,11 @@ Possible features: > >> - KVM_ARM_VCPU_PTRAUTH_ADDRESS: Enables Address Pointer authentication > >> for the CPU and supported only on arm64 architecture. > >> Must be requested if KVM_ARM_VCPU_PTRAUTH_GENERIC is also requested. > >>+ Depends on KVM_CAP_ARM_PTRAUTH_ADDRESS. > > > >What if KVM_CAP_ARM_PTRAUTH_ADDRESS is absent and > >KVM_ARM_VCPU_PTRAUTH_GENERIC is requested? By these rules, we have a > >contradiction: userspace both must request and must not request > >KVM_ARM_VCPU_PTRAUTH_ADDRESS. > > > >We could qualify as follows: > > > > Depends on KVM_CAP_ARM_PTRAUTH_ADDRESS. > > Must be requested if KVM_CAP_ARM_PTRAUTH_ADDRESS is present and > > KVM_ARM_VCPU_PTRAUTH_GENERIC is also requested. > ok agree. This makes it clear. [*] > >> - KVM_ARM_VCPU_PTRAUTH_GENERIC: Enables Generic Pointer authentication > >> for the CPU and supported only on arm64 architecture. > >> Must be requested if KVM_ARM_VCPU_PTRAUTH_ADDRESS is also requested. > >>+ Depends on KVM_CAP_ARM_PTRAUTH_GENERIC. > > > >Similarly. > > > >Or, we go back to having a single cap and a single feature, and add > >more caps/features later on if we decide it's possible to support > >address/generic auth separately later on. > > > >Otherwise, we end up with complex rules that can't be tested. This is a > >high price to pay for forwards compatibility: userspace's conformance to > >the rules can't be fully tested, so there's a fair chance it won't work > >properly anyway when hardware/KVM with just one auth type appears. > > > >[...] > > > >Thoughts? > I agree that single cpufeature/capability is a simple solution to implement. > The bifurcation of feature was done to reflect the different ID register > split up. > > But the h/w implementation provides a same EL2 exception trap for both the > features and hence current implementation ties both of the features > together. I guess in future if this is limitation goes away then one auth > type is possible. Here I am not sure if the future h/w will retain this > merged exception trap and add 2 new separate exception trap in addition to > it. > > I guess it will be probably simple split-up of this merged exception trap. > In this case there won't be any ABI change required as per current > implementation. OK, I'm not opposed to keeping the ABI as-is, with the above clarification [*] spelled out appropriately for both cases. Alternatively, or in addition, we could say something like: "If KVM_CAP_ARM_PTRAUTH_ADDRESS and KVM_CAP_ARM_PTRAUTH_GENERIC are both present, then both KVM_ARM_VCPU_PTRAUTH_ADDRESS and KVM_ARM_VCPU_PTRAUTH_GENERIC must be requested or neither must be requested." Cheers ---Dave