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=-3.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, USER_AGENT_NEOMUTT 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 10397C04EB8 for ; Mon, 10 Dec 2018 11:41:18 +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 D4EB420880 for ; Mon, 10 Dec 2018 11:41:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="nlRF4pHc" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D4EB420880 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:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=JiPCjJRq4JfIXffujg6durJii83gpfzdigFQ7/BSzF0=; b=nlRF4pHc5DAp/S i70sgI431lwRGk9NnC0s8RGqeO3fTZTRld3Pdw203d3hf09R1BQN2V67etVQgNVnCenKP6f4FIE6P dApXq/cX6zg4mB816ZBYePX9sYtM/SIgeqU6hyniuPttRnOOmaezhCQTfJbLhVfeE0V1oiNIjdMge RXz5RqNrmLRs6UFKkPU9zK+Yx0RfV/FV+0pjri3Qj4LL3GqTaZb04l6NW/z1zlcJ8J3su5cqgR0ar 1j3c19jZZ4ZRmfFgKb0N2Wcs7STxle2YK0xuwQohUwnKRp8IOcDxr5+pShNercWN47SYhbX3EC9dt LCyGo2hf8KiIkKrEfOzg==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1gWJw2-0003Zb-24; Mon, 10 Dec 2018 11:41:14 +0000 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70] helo=foss.arm.com) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1gWJvq-0003Lg-OW for linux-arm-kernel@lists.infradead.org; Mon, 10 Dec 2018 11:41:08 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 775D215AB; Mon, 10 Dec 2018 03:40:51 -0800 (PST) Received: from lakrids.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1FC043F6A8; Mon, 10 Dec 2018 03:40:49 -0800 (PST) Date: Mon, 10 Dec 2018 11:40:47 +0000 From: Mark Rutland To: Steven Price Subject: Re: [PATCH 00/12] arm64: Paravirtualized time support Message-ID: <20181210114047.tifwh6ilwzphsbqy@lakrids.cambridge.arm.com> References: <20181128144527.44710-1-steven.price@arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20181128144527.44710-1-steven.price@arm.com> User-Agent: NeoMutt/20170113 (1.7.2) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20181210_034103_353680_99944FAB X-CRM114-Status: GOOD ( 17.00 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Marc Zyngier , Catalin Marinas , Will Deacon , Christoffer Dall , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org 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 Wed, Nov 28, 2018 at 02:45:15PM +0000, Steven Price wrote: > This series add support for paravirtualized time for Arm64 guests and > KVM hosts following the specification in Arm's document DEN 0057A: > > https://developer.arm.com/docs/den0057/a > > It implements support for Live Physical Time (LPT) which provides the > guest with a method to derive a stable counter of time during which the > guest is executing even when the guest is being migrated between hosts > with different physical counter frequencies. > > It also implements support for stolen time, allowing the guest to > identify time when it is forcibly not executing. I know that stolen time reporting is important, and I think that we definitely want to pick up that part of the spec (once it is published in some non-draft form). However, I am very concerned with the pv-freq part of LPT, and I'd like to avoid that if at all possible. I say that because: * By design, it breaks architectural guarantees from the PoV of SW in the guest. A VM may host multiple SW agents serially (e.g. when booting, or across kexec), or concurrently (e.g. Linux w/ EFI runtime services), and the host has no way to tell whether all software in the guest will function correctly. Due to this, it's not possible to have a guest opt-in to the architecturally-broken timekeeping. Existing guests will not work correctly once pv-freq is in use, and if configured without pv-freq (or if the guest fails to discover pv-freq for any reason), the administrator may encounter anything between subtle breakage or fatally incorrect timekeeping. There's plenty of SW agents other than Linux which runs in a guest, which would need to be updated to handle pv-freq, e.g. GRUB, *BSD, iPXE. Given this, I think that this is going to lead to subtle breakage in real-world scenarios. * It is (necessarily) invasive to the low-level arch timer code. This is unfortunate, and I strongly suspect this is going to be an area with long-term subtle breakage. * It's not clear to me how strongly people need this. My understanding is that datacenters would run largely homogeneous platforms. I suspect large datacenters which would use migration are in a position to mandate a standard timer frequency from their OEMs or SiPs. I strongly believe that an architectural fix (e.g. in-hw scaling) would be the better solution. I understand that LPT is supposed to account for time lost during the migration. Can we account for this without pv-freq? e.g. is it possible to account for this in the same way as stolen time? Thanks, Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel