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=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 D6F9EC433FE for ; Tue, 8 Dec 2020 11:25:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A061B239E5 for ; Tue, 8 Dec 2020 11:25:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729149AbgLHLZx (ORCPT ); Tue, 8 Dec 2020 06:25:53 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:47177 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728983AbgLHLZw (ORCPT ); Tue, 8 Dec 2020 06:25:52 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1607426665; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=LtCo72TfeKmFl6M1Uz4zaJ8a+OPs0eUphJ7mDeWhrv8=; b=Q4dGfZEtcwmzDVJ6GFmoaaNZg25Q3/bXuuJpLXHU9VVmAQZ8v9bSppHBdxbPFPpWOL71Ua IyQNkKCmREQKiOfYPHGt2j4/cXOMna+jYZJhVNm7TNbuIikNwGu6GG9iMQQR7LbqQmXAyO yOEE2YOZ7ZqmrCWqBxvSoHLy/jIDWyY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-209-u54GPkF2Obyv-VNJM2ywhg-1; Tue, 08 Dec 2020 06:24:23 -0500 X-MC-Unique: u54GPkF2Obyv-VNJM2ywhg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 4034E180A086; Tue, 8 Dec 2020 11:24:20 +0000 (UTC) Received: from starship (unknown [10.35.206.133]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8BE3919D80; Tue, 8 Dec 2020 11:24:13 +0000 (UTC) Message-ID: <69a4b9888a179529607d22615caa647b5fbf051d.camel@redhat.com> Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE From: Maxim Levitsky To: Andy Lutomirski Cc: Thomas Gleixner , kvm@vger.kernel.org, "H. Peter Anvin" , Paolo Bonzini , Jonathan Corbet , Jim Mattson , Wanpeng Li , "open list:KERNEL SELFTEST FRAMEWORK" , Vitaly Kuznetsov , Marcelo Tosatti , Sean Christopherson , open list , Ingo Molnar , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Joerg Roedel , Borislav Petkov , Shuah Khan , Andrew Jones , Oliver Upton , "open list:DOCUMENTATION" Date: Tue, 08 Dec 2020 13:24:11 +0200 In-Reply-To: <885C1725-B479-47F6-B08D-A7181637A80A@amacapital.net> References: <636fecc20b0143128b484f159ff795ff65d05b82.camel@redhat.com> <885C1725-B479-47F6-B08D-A7181637A80A@amacapital.net> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.5 (3.36.5-1.fc32) MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2020-12-07 at 10:04 -0800, Andy Lutomirski wrote: > > On Dec 7, 2020, at 9:00 AM, Maxim Levitsky wrote: > > > > On Mon, 2020-12-07 at 08:53 -0800, Andy Lutomirski wrote: > > > > > On Dec 7, 2020, at 8:38 AM, Thomas Gleixner wrote: > > > > > > > > On Mon, Dec 07 2020 at 14:16, Maxim Levitsky wrote: > > > > > > On Sun, 2020-12-06 at 17:19 +0100, Thomas Gleixner wrote: > > > > > > From a timekeeping POV and the guests expectation of TSC this is > > > > > > fundamentally wrong: > > > > > > > > > > > > tscguest = scaled(hosttsc) + offset > > > > > > > > > > > > The TSC has to be viewed systemwide and not per CPU. It's systemwide > > > > > > used for timekeeping and for that to work it has to be synchronized. > > > > > > > > > > > > Why would this be different on virt? Just because it's virt or what? > > > > > > > > > > > > Migration is a guest wide thing and you're not migrating single vCPUs. > > > > > > > > > > > > This hackery just papers over he underlying design fail that KVM looks > > > > > > at the TSC per vCPU which is the root cause and that needs to be fixed. > > > > > > > > > > I don't disagree with you. > > > > > As far as I know the main reasons that kvm tracks TSC per guest are > > > > > > > > > > 1. cases when host tsc is not stable > > > > > (hopefully rare now, and I don't mind making > > > > > the new API just refuse to work when this is detected, and revert to old way > > > > > of doing things). > > > > > > > > That's a trainwreck to begin with and I really would just not support it > > > > for anything new which aims to be more precise and correct. TSC has > > > > become pretty reliable over the years. > > > > > > > > > 2. (theoretical) ability of the guest to introduce per core tsc offfset > > > > > by either using TSC_ADJUST (for which I got recently an idea to stop > > > > > advertising this feature to the guest), or writing TSC directly which > > > > > is allowed by Intel's PRM: > > > > > > > > For anything halfways modern the write to TSC is reflected in TSC_ADJUST > > > > which means you get the precise offset. > > > > > > > > The general principle still applies from a system POV. > > > > > > > > TSC base (systemwide view) - The sane case > > > > > > > > TSC CPU = TSC base + TSC_ADJUST > > > > > > > > The guest TSC base is a per guest constant offset to the host TSC. > > > > > > > > TSC guest base = TSC host base + guest base offset > > > > > > > > If the guest want's this different per vCPU by writing to the MSR or to > > > > TSC_ADJUST then you still can have a per vCPU offset in TSC_ADJUST which > > > > is the offset to the TSC base of the guest. > > > > > > How about, if the guest wants to write TSC_ADJUST, it can turn off all paravirt features and keep both pieces? > > > > > > > This is one of the things I had in mind recently. > > > > Even better, we can stop advertising TSC_ADJUST in CPUID to the guest > > and forbid it from writing it at all. > > Seems reasonable to me. > > It also seems okay for some MSRs to stop working after the guest enabled new PV timekeeping. This is a very good idea! > > I do have a feature request, though: IMO it would be quite nifty if the new kvmclock structure could also expose NTP corrections. In other words, if you could expose enough info to calculate CLOCK_MONOTONIC_RAW, CLOCK_MONOTONIC, and CLOCK_REALTIME, then we could have paravirt NTP. > > Bonus points if whatever you do for CLOCK_REALTIME also exposes leap seconds in a race free way :). But I suppose that just exposing TAI and letting the guest deal with the TAI - UTC offset itself would get the job done just fine. This is a good idea too. As I understand it, this gives a justification to a new kvmclock purpose, which wouldn't be focused anymore on correcting the tsc shortcomings (unstable/unscalable tsc), but more on things like that. I like that idea. Best regards, Maxim Levitsky