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

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Date:   Thu, 04 Oct 2018 09:54:45 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        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

Marcelo Tosatti <mtosatti@...hat.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).
- 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?

-- 
Vitaly