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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,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 0896BC3A5A2 for ; Fri, 23 Aug 2019 10:34:03 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id CEF022166E for ; Fri, 23 Aug 2019 10:34:02 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="f/Syri/v" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CEF022166E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date: Message-ID:From:References:To:Subject:Reply-To:Content-ID:Content-Description :Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=ATaWGHMZeUqq9eKblUB9pzCRkgUpU5wQBENxtEjF7lE=; b=f/Syri/v5lmIhT U3PtVEYBTol7g5hX1y1dxUb7Ev4oB3Dsek5iC65iygldAsO9j6J4vkLZ1mWbfsUsw3mi7BUtLVhbG 2HxA0raxM5chuOspbzSBqHGe6EuYLiVN7vVG6nQye1G8HmgtJP5UoCduMEVQL8ezcrto04m0se+XB ziflCi3jWSgMt7T+WsIaz/RVymEm9bfogNrX7PgDTdAMaBq/9s1hdzpKr9GZ/+O2XRDSAVMbdx77F saMhVIuCxqabargT7sE8o+IelNt8JkKry1hQdy11zT82NdpDDRvHC/82p9CnnUbNfXLwoOIsT98Up HEYJiIuiz044j/7b3HVA==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1i16tI-0004sp-Rk; Fri, 23 Aug 2019 10:33:56 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1i16tG-0004sO-Cl for linux-arm-kernel@lists.infradead.org; Fri, 23 Aug 2019 10:33:55 +0000 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 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-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190823_033354_519246_0A436E94 X-CRM114-Status: GOOD ( 21.87 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , kvm@vger.kernel.org, Suzuki K Pouloze , Marc Zyngier , linux-doc@vger.kernel.org, Russell King , linux-kernel@vger.kernel.org, James Morse , linux-arm-kernel@lists.infradead.org, Catalin Marinas , Paolo Bonzini , Will Deacon , kvmarm@lists.cs.columbia.edu, Julien Thierry Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel