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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_2 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 86916C31E40 for ; Sat, 3 Aug 2019 18:05:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5E581206A2 for ; Sat, 3 Aug 2019 18:05:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1564855537; bh=NvLz+V5uiaEIZFYvMNrg/1o0YQBwMMp7TsyzgawILvs=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=PIyaiGak/ffYIvTjhBlUAiQzk/Cu7Rhs59J1e4IyTwGckPHHrRtxliKFp9rObTnGU 7QeUwyRIEw2CCgFWckZCdXr1ofVr6sVi6NbHuyLiAYiqJgW8TejJp1rNld1QP5Wfq1 ZorFNGR8x8p/NjoBcBZMEQ8YmKPS575gNZ6Ob6Zw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728518AbfHCSFd (ORCPT ); Sat, 3 Aug 2019 14:05:33 -0400 Received: from inca-roads.misterjones.org ([213.251.177.50]:48507 "EHLO inca-roads.misterjones.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728366AbfHCSFd (ORCPT ); Sat, 3 Aug 2019 14:05:33 -0400 Received: from 78.163-31-62.static.virginmediabusiness.co.uk ([62.31.163.78] helo=why) by cheepnis.misterjones.org with esmtpsa (TLSv1.2:AES256-GCM-SHA384:256) (Exim 4.80) (envelope-from ) id 1htyPF-0002A1-0t; Sat, 03 Aug 2019 20:05:25 +0200 Date: Sat, 3 Aug 2019 19:05:22 +0100 From: Marc Zyngier To: Steven Price Cc: Catalin Marinas , Paolo Bonzini , Radim =?UTF-8?B?S3LEjW3DocWZ?= , Russell King , Will Deacon , James Morse , Julien Thierry , Suzuki K Pouloze , kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/9] arm64: Stolen time support Message-ID: <20190803190522.5fec8f7d@why> In-Reply-To: <20190802145017.42543-1-steven.price@arm.com> References: <20190802145017.42543-1-steven.price@arm.com> Organization: Approximate X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-SA-Exim-Connect-IP: 62.31.163.78 X-SA-Exim-Rcpt-To: steven.price@arm.com, catalin.marinas@arm.com, pbonzini@redhat.com, rkrcmar@redhat.com, linux@armlinux.org.uk, will@kernel.org, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com, kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on cheepnis.misterjones.org); SAEximRunCond expanded to false Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, 2 Aug 2019 15:50:08 +0100 Steven Price wrote: Hi Steven, > 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 stolen time, allowing the guest to > identify time when it is forcibly not executing. > > It doesn't implement support for Live Physical Time (LPT) as there are > some concerns about the overheads and approach in the above > specification, and I expect an updated version of the specification to > be released soon with just the stolen time parts. Thanks for posting this. My current concern with this series is around the fact that we allocate memory from the kernel on behalf of the guest. It is the first example of such thing in the ARM port, and I can't really say I'm fond of it. x86 seems to get away with it by having the memory allocated from userspace, why I tend to like more. Yes, put_user is more expensive than a straight store, but this isn't done too often either. What is the rational for your current approach? Thanks, M. -- Without deviation from the norm, progress is not possible.