linux-kernel - Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK

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Date:   Thu, 4 Oct 2018 07:04:38 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Matt Rickard <matt@...trans.com.au>,
        Stephen Boyd <sboyd@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Florian Weimer <fweimer@...hat.com>,
        KY Srinivasan <kys@...rosoft.com>,
        devel@...uxdriverproject.org,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, Juergen Gross <jgross@...e.com>
Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support


> On Oct 4, 2018, at 5:00 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
>> On 04/10/2018 09:54, Vitaly Kuznetsov wrote:
>> - Check if pure TSC can be used on SkyLake+ systems (where TSC scaling
>> is supported).
> 
> Not if you want to migrate to pre-Skylake systems.
> 
>> - Check if non-masterclock mode is still needed. E.g. HyperV's TSC page
>> clocksource is a single page for the whole VM, not a per-cpu thing. Can
>> we think that all the buggy hardware is already gone?
> 
> No. :(  We still get reports whenever we break 2007-2008 hardware.
> 
> 

Does the KVM non-masterclock mode actually help?  It’s not clear to me exactly how it’s supposed to work, but it seems like it’s trying to expose per-vCPU adjustments to the guest. Which is dubious at best, since the guest can’t validly use them for anything other than sched_clock, since they aren’t fully corrected by anything KVM can do.