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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 64F66C3A5A2 for ; Fri, 23 Aug 2019 10:33:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3D99C2166E for ; Fri, 23 Aug 2019 10:33:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391112AbfHWKdz (ORCPT ); Fri, 23 Aug 2019 06:33:55 -0400 Received: from foss.arm.com ([217.140.110.172]:59664 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732974AbfHWKdy (ORCPT ); Fri, 23 Aug 2019 06:33:54 -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 CF842337; Fri, 23 Aug 2019 03:33:53 -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 4123A3F246; Fri, 23 Aug 2019 03:33:51 -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> <20190822162449.GF25467@linux.intel.com> From: Steven Price Message-ID: <691598cf-284d-5156-c15f-78d363b9f18e@arm.com> Date: Fri, 23 Aug 2019 11:33:49 +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: <20190822162449.GF25467@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 22/08/2019 17:24, Sean Christopherson wrote: > On Thu, Aug 22, 2019 at 04:46:10PM +0100, Steven Price wrote: >> 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). > > Thanks for the explanation. I assumed that's what you were referring to, > but wanted to double check. > >> __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). > > I don't think kvm_put_guest() belongs in generic code, at least not with > the current changelog explanation about it providing single-copy atomic > semantics. AFAICT, the single-copy thing is very much an arm64 > implementation detail, e.g. the vast majority of 32-bit architectures, > including x86, do not provide any guarantees, and x86-64 generates more > or less the same code for put_user() and __copy_to_user() for 8-byte and > smaller accesses. > > As an alternative to kvm_put_guest() entirely, is it an option to change > arm64's raw_copy_to_user() to redirect to __put_user() for sizes that are > constant at compile time and can be handled by __put_user()? That would > allow using kvm_write_guest() to update stolen time, albeit with > arguably an even bigger dependency on the uaccess implementation details. I think it's important to in some way ensure that the desire that this is a single write is shown. copy_to_user() is effectively "setup();memcpy();finish();" and while a good memcpy() implementation would be identical to put_user() there's a lot more room for this being broken in the future by changes to the memcpy() implementation. (And I don't want to require that memcpy() has to detect this case). One suggestion is to call it something like kvm_put_guest_atomic() to reflect the atomicity requirement. Presumably that would be based on a new put_user_atomic() which architectures could override as necessary if put_user() doesn't provide the necessary guarantees. Steve