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_PASS,URIBL_BLOCKED,USER_AGENT_MUTT 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 239C6C64EB8 for ; Wed, 3 Oct 2018 19:01:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EA276213A2 for ; Wed, 3 Oct 2018 19:01:08 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EA276213A2 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727206AbeJDBup (ORCPT ); Wed, 3 Oct 2018 21:50:45 -0400 Received: from mx1.redhat.com ([209.132.183.28]:51772 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726950AbeJDBup (ORCPT ); Wed, 3 Oct 2018 21:50:45 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id C87DC81DEB; Wed, 3 Oct 2018 19:01:05 +0000 (UTC) Received: from amt.cnet (ovpn-112-2.gru2.redhat.com [10.97.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTP id DFEB0648C3; Wed, 3 Oct 2018 19:01:04 +0000 (UTC) Received: from amt.cnet (localhost [127.0.0.1]) by amt.cnet (Postfix) with ESMTP id 54CD7105148; Wed, 3 Oct 2018 16:00:46 -0300 (BRT) Received: (from marcelo@localhost) by amt.cnet (8.14.7/8.14.7/Submit) id w93J0UXc023167; Wed, 3 Oct 2018 16:00:30 -0300 Date: Wed, 3 Oct 2018 16:00:29 -0300 From: Marcelo Tosatti To: Andy Lutomirski Cc: Thomas Gleixner , Paolo Bonzini , Radim Krcmar , Wanpeng Li , LKML , X86 ML , Peter Zijlstra , Matt Rickard , Stephen Boyd , John Stultz , Florian Weimer , KY Srinivasan , Vitaly Kuznetsov , devel@linuxdriverproject.org, Linux Virtualization , Arnd Bergmann , Juergen Gross Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support Message-ID: <20181003190026.GB21381@amt.cnet> References: <20180914125006.349747096@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Wed, 03 Oct 2018 19:01:06 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 02, 2018 at 10:15:49PM -0700, Andy Lutomirski wrote: > Hi Vitaly, Paolo, Radim, etc., > > On Fri, Sep 14, 2018 at 5:52 AM Thomas Gleixner wrote: > > > > Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime() > > implementation, which extended the clockid switch case and added yet > > another slightly different copy of the same code. > > > > Especially the extended switch case is problematic as the compiler tends to > > generate a jump table which then requires to use retpolines. If jump tables > > are disabled it adds yet another conditional to the existing maze. > > > > This series takes a different approach by consolidating the almost > > identical functions into one implementation for high resolution clocks and > > one for the coarse grained clock ids by storing the base data for each > > clock id in an array which is indexed by the clock id. > > > > I was trying to understand more of the implications of this patch > series, and I was again reminded that there is an entire extra copy of > the vclock reading code in arch/x86/kvm/x86.c. And the purpose of > that code is very, very opaque. > > Can one of you explain what the code is even doing? From a couple of > attempts to read through it, it's a whole bunch of > probably-extremely-buggy code that, Yes, probably. > drumroll please, tries to atomically read the TSC value and the time. And decide whether the > result is "based on the TSC". I think "based on the TSC" refers to whether TSC clocksource is being used. > And then synthesizes a TSC-to-ns > multiplier and shift, based on *something other than the actual > multiply and shift used*. > > IOW, unless I'm totally misunderstanding it, the code digs into the > private arch clocksource data intended for the vDSO, uses a poorly > maintained copy of the vDSO code to read the time (instead of doing > the sane thing and using the kernel interfaces for this), and > propagates a totally made up copy to the guest. I posted kernel interfaces for this, and it was suggested to instead write a "in-kernel user of pvclock data". If you can get kernel interfaces to replace that, go for it. I prefer kernel interfaces as well. > And gets it entirely > wrong when doing nested virt, since, unless there's some secret in > this maze, it doesn't acutlaly use the scaling factor from the host > when it tells the guest what to do. > > I am really, seriously tempted to send a patch to simply delete all > this code. If your patch which deletes the code gets the necessary features right, sure, go for it. > The correct way to do it is to hook Can you expand on the correct way to do it? > And I don't see how it's even possible to pass kvmclock correctly to > the L2 guest when L0 is hyperv. KVM could pass *hyperv's* clock, but > L1 isn't notified when the data structure changes, so how the heck is > it supposed to update the kvmclock structure? I don't parse your question. > > --Andy