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,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 DDDD7C07E94 for ; Fri, 4 Jun 2021 15:34:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C16A961406 for ; Fri, 4 Jun 2021 15:34:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230385AbhFDPge (ORCPT ); Fri, 4 Jun 2021 11:36:34 -0400 Received: from mail.kernel.org ([198.145.29.99]:35550 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229675AbhFDPge (ORCPT ); Fri, 4 Jun 2021 11:36:34 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id CC37361402; Fri, 4 Jun 2021 15:34:44 +0000 (UTC) Date: Fri, 4 Jun 2021 16:34:42 +0100 From: Catalin Marinas To: Steven Price 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 Subject: Re: [PATCH v13 7/8] KVM: arm64: ioctl to fetch/store tags in a guest Message-ID: <20210604153441.GG31173@arm.com> References: <20210524104513.13258-1-steven.price@arm.com> <20210524104513.13258-8-steven.price@arm.com> <20210603171336.GH20338@arm.com> <02c7682e-5fb6-29eb-9105-02e3521756a2@arm.com> <20210604114233.GE31173@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 04, 2021 at 02:09:50PM +0100, Steven Price wrote: > On 04/06/2021 12:42, Catalin Marinas wrote: > > On Fri, Jun 04, 2021 at 12:15:56PM +0100, Steven Price wrote: > >> On 03/06/2021 18:13, Catalin Marinas wrote: > >>> On Mon, May 24, 2021 at 11:45:12AM +0100, Steven Price wrote: > >>>> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h > >>>> index 24223adae150..b3edde68bc3e 100644 > >>>> --- a/arch/arm64/include/uapi/asm/kvm.h > >>>> +++ b/arch/arm64/include/uapi/asm/kvm.h > >>>> @@ -184,6 +184,17 @@ struct kvm_vcpu_events { > >>>> __u32 reserved[12]; > >>>> }; > >>>> > >>>> +struct kvm_arm_copy_mte_tags { > >>>> + __u64 guest_ipa; > >>>> + __u64 length; > >>>> + void __user *addr; > >>>> + __u64 flags; > >>>> + __u64 reserved[2]; > >>>> +}; > >>>> + > >>>> +#define KVM_ARM_TAGS_TO_GUEST 0 > >>>> +#define KVM_ARM_TAGS_FROM_GUEST 1 > >>>> + > >>>> /* If you need to interpret the index values, here is the key: */ > >>>> #define KVM_REG_ARM_COPROC_MASK 0x000000000FFF0000 > >>>> #define KVM_REG_ARM_COPROC_SHIFT 16 > >>>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c > >>>> index e89a5e275e25..baa33359e477 100644 > >>>> --- a/arch/arm64/kvm/arm.c > >>>> +++ b/arch/arm64/kvm/arm.c > >>>> @@ -1345,6 +1345,13 @@ long kvm_arch_vm_ioctl(struct file *filp, > >>>> > >>>> return 0; > >>>> } > >>>> + case KVM_ARM_MTE_COPY_TAGS: { > >>>> + struct kvm_arm_copy_mte_tags copy_tags; > >>>> + > >>>> + if (copy_from_user(©_tags, argp, sizeof(copy_tags))) > >>>> + return -EFAULT; > >>>> + return kvm_vm_ioctl_mte_copy_tags(kvm, ©_tags); > >>>> + } > >>> > >>> I wonder whether we need an update of the user structure following a > >>> fault, like how much was copied etc. In case of an error, some tags were > >>> copied and the VMM may want to skip the page before continuing. But here > >>> there's no such information provided. > >>> > >>> On the ptrace interface, we return 0 on the syscall if any bytes were > >>> copied and update iov_len to such number. Maybe you want to still return > >>> an error here but updating copy_tags.length would be nice (and, of > >>> course, a copy_to_user() back). > >> > >> Good idea - as you suggest I'll make it update length with the number of > >> bytes not processed. Although in general I think we're expecting the VMM > >> to know where the memory is so this is more of a programming error - but > >> could still be useful for debugging. > > > > Or update it to the number of bytes copied to be consistent with > > ptrace()'s iov.len. On success, the structure is effectively left > > unchanged. > > I was avoiding that because it confuses the error code when the initial > copy_from_user() fails. In that case the structure is clearly unchanged, > so you can only tell from a -EFAULT return that nothing happened. By > returning the number of bytes left you can return an error code along > with the information that the copy only half completed. > > It also seems cleaner to leave the structure unchanged if e.g. the flags > or reserved fields are invalid rather than having to set length=0 to > signal that nothing was done. > > Although I do feel like arguing whether to use a ptrace() interface or a > copy_{to,from}_user() interface is somewhat ridiculous considering > neither are exactly considered good. > > Rather than changing the structure we could return either an error code > (if nothing was copied) or the number of bytes left. That way ioctl()==0 > means complete success, >0 means partial success and <0 means complete > failure and provides a detailed error code. The ioctl() can be repeated > (with adjusted pointers) if it returns >0 and a detailed error is needed. That would be more like read/write (nearly, those always return the amount copied). Anyway, I don't have any strong preference, I'll leave the details up to you as long as there is some indication of how much was copied or left. -- Catalin