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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 DD5EAC433ED for ; Fri, 7 May 2021 09:44:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A1B2A6145D for ; Fri, 7 May 2021 09:44:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236812AbhEGJpp (ORCPT ); Fri, 7 May 2021 05:45:45 -0400 Received: from foss.arm.com ([217.140.110.172]:52474 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234506AbhEGJpk (ORCPT ); Fri, 7 May 2021 05:45:40 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id CF03C106F; Fri, 7 May 2021 02:44:40 -0700 (PDT) Received: from [192.168.1.179] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 534693F718; Fri, 7 May 2021 02:44:38 -0700 (PDT) Subject: Re: [PATCH v11 5/6] KVM: arm64: ioctl to fetch/store tags in a guest To: Catalin Marinas Cc: Marc Zyngier , Will Deacon , James Morse , Julien Thierry , Suzuki K Poulose , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Dave Martin , Mark Rutland , Thomas Gleixner , qemu-devel@nongnu.org, Juan Quintela , "Dr. David Alan Gilbert" , Richard Henderson , Peter Maydell , Haibo Xu , Andrew Jones References: <20210416154309.22129-1-steven.price@arm.com> <20210416154309.22129-6-steven.price@arm.com> <20210427175844.GB17872@arm.com> <340d35c2-46ed-35ea-43fa-e5cb64c27230@arm.com> From: Steven Price Message-ID: <25c85740-0119-549e-6ddb-aea69c5efc76@arm.com> Date: Fri, 7 May 2021 10:44:33 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/05/2021 18:44, Catalin Marinas wrote: > On Thu, Apr 29, 2021 at 05:06:07PM +0100, Steven Price wrote: >> On 27/04/2021 18:58, Catalin Marinas wrote: >>> On Fri, Apr 16, 2021 at 04:43:08PM +0100, Steven Price wrote: >>>> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h >>>> index 24223adae150..2b85a047c37d 100644 >>>> --- a/arch/arm64/include/uapi/asm/kvm.h >>>> +++ b/arch/arm64/include/uapi/asm/kvm.h >>>> @@ -184,6 +184,20 @@ struct kvm_vcpu_events { >>>> __u32 reserved[12]; >>>> }; >>>> +struct kvm_arm_copy_mte_tags { >>>> + __u64 guest_ipa; >>>> + __u64 length; >>>> + union { >>>> + void __user *addr; >>>> + __u64 padding; >>>> + }; >>>> + __u64 flags; >>>> + __u64 reserved[2]; >>>> +}; > [...] >>> Maybe add the two reserved >>> values to the union in case we want to store something else in the >>> future. >> >> I'm not sure what you mean here. What would the reserved fields be unioned >> with? And surely they are no longer reserved in that case? > > In case you want to keep the structure size the same for future > expansion and the expansion only happens via the union, you'd add some > padding in there just in case. We do this for struct siginfo with an > _si_pad[] array in the union. > Ah I see what you mean. In this case "padding" is just a sizer to ensure that flags is always the same alignment - it's not intended to be used. As I noted previously though it's completely pointless as this only on arm64 and even 32 bit Arm would naturally align the following __u64. reserved[] is for expansion and I guess we could have a union over the whole struct (like siginfo) but I think it's generally clearer to just spell out the reserved fields at the end of the struct. TLDR; the union will be gone along with "padding" in the next version. "reserved" remains at the end of the struct for future use. Thanks, Steve