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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 55D3FC3A5A1 for ; Thu, 22 Aug 2019 15:46:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 383F523404 for ; Thu, 22 Aug 2019 15:46:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387689AbfHVPqO (ORCPT ); Thu, 22 Aug 2019 11:46:14 -0400 Received: from foss.arm.com ([217.140.110.172]:48276 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730741AbfHVPqO (ORCPT ); Thu, 22 Aug 2019 11:46:14 -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 A80BA337; Thu, 22 Aug 2019 08:46:13 -0700 (PDT) Received: from [10.1.196.133] (e112269-lin.cambridge.arm.com [10.1.196.133]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B7BD53F718; Thu, 22 Aug 2019 08:46:11 -0700 (PDT) Subject: Re: [PATCH v3 04/10] KVM: Implement kvm_put_guest() To: Sean Christopherson Cc: Mark Rutland , kvm@vger.kernel.org, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Marc Zyngier , Suzuki K Pouloze , linux-doc@vger.kernel.org, Russell King , linux-kernel@vger.kernel.org, James Morse , Julien Thierry , Catalin Marinas , Paolo Bonzini , Will Deacon , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org References: <20190821153656.33429-1-steven.price@arm.com> <20190821153656.33429-5-steven.price@arm.com> <20190822152854.GE25467@linux.intel.com> From: Steven Price Message-ID: Date: Thu, 22 Aug 2019 16:46:10 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190822152854.GE25467@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 22/08/2019 16:28, Sean Christopherson wrote: > On Wed, Aug 21, 2019 at 04:36:50PM +0100, Steven Price wrote: >> kvm_put_guest() is analogous to put_user() - it writes a single value to >> the guest physical address. The implementation is built upon put_user() >> and so it has the same single copy atomic properties. > > What you mean by "single copy atomic"? I.e. what guarantees does > put_user() provide that __copy_to_user() does not? Single-copy atomicity is defined by the Arm architecture[1] and I'm not going to try to go into the full details here, so this is a summary. For the sake of this feature what we care about is that the value written/read cannot be "torn". In other words if there is a read (in this case from another VCPU) that is racing with the write then the read will either get the old value or the new value. It cannot return a mixture. (This is of course assuming that the read is using a single-copy atomic safe method). __copy_to_user() is implemented as a memcpy() and as such cannot provide single-copy atomicity in the general case (the buffer could easily be bigger than the architecture can guarantee). put_user() on the other hand is implemented (on arm64) as an explicit store instruction and therefore is guaranteed by the architecture to be single-copy atomic (i.e. another CPU cannot see a half-written value). Steve [1] https://static.docs.arm.com/ddi0487/ea/DDI0487E_a_armv8_arm.pdf#page=110