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Date:   Fri, 5 Oct 2018 18:18:41 -0300
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>,
        Wanpeng Li <kernellwp@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Matt Rickard <matt@softrans.com.au>,
        Stephen Boyd <sboyd@kernel.org>,
        John Stultz <john.stultz@linaro.org>,
        Florian Weimer <fweimer@redhat.com>,
        KY Srinivasan <kys@microsoft.com>,
        devel@linuxdriverproject.org,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>, Juergen Gross <jgross@suse.com>
Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI
 support
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On Thu, Oct 04, 2018 at 09:54:45AM +0200, Vitaly Kuznetsov wrote:
> Marcelo Tosatti <mtosatti@redhat.com> writes:
> 
> > On Wed, Oct 03, 2018 at 11:22:58AM +0200, Vitaly Kuznetsov wrote:
> >> 
> >> There is a very long history of different (hardware) issues Marcelo was
> >> fighting with and the current code is the survived Frankenstein.
> >
> > Right, the code has to handle different TSC modes.
> >
> >>  E.g. it
> >> is very, very unclear what "catchup", "always catchup" and
> >> masterclock-less mode in general are and if we still need them.
> >
> > Catchup means handling exposed (to guest) TSC frequency smaller than
> > HW TSC frequency.
> >
> > That is "frankenstein" code, could be removed.
> >
> >> That said I'm all for simplification. I'm not sure if we still need to
> >> care about buggy hardware though.
> >
> > What simplification is that again? 
> >
> 
> I was hoping to hear this from you :-) If I am to suggest how we can
> move forward I'd propose:
> - Check if pure TSC can be used on SkyLake+ systems (where TSC scaling
> is supported).

In that case just use TSC clocksource on the guest directly: i am
writing code for that now (its faster than pvclock syscall).

> - 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?

As Peter mentioned, non sync TSC hardware might exist in the future. 
And older hardware must be supported.