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

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Message-Id: <8C316427-8BEC-4979-8AB2-5E385066BB6F@amacapital.net>
Date:   Wed, 3 Oct 2018 07:20:48 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        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



> On Oct 3, 2018, at 5:01 AM, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> 
> Andy Lutomirski <luto@...capital.net> writes:
> 
>>> On Oct 3, 2018, at 2:22 AM, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>>> 
>>> Andy Lutomirski <luto@...nel.org> writes:
>>> 
>>>> Hi Vitaly, Paolo, Radim, etc.,
>>>> 
>>> The notification you're talking about exists, it is called
>>> Reenligntenment, see 0092e4346f49 "x86/kvm: Support Hyper-V
>>> reenlightenment"). When TSC page changes (and this only happens when L1
>>> is migrated to a different host with a different TSC frequency and TSC
>>> scaling is not supported by the CPU) we receive an interrupt in L1 (at
>>> this moment all TSC accesses are emulated which guarantees the
>>> correctness of the readings), pause all L2 guests, update their kvmclock
>>> structures with new data (we already know the new TSC frequency) and
>>> then tell L0 that we're done and it can stop emulating TSC accesses.
>> 
>> That’s delightful!  Does the emulation magic also work for L1 user
>> mode?
> 
> As far as I understand - yes, all rdtsc* calls will trap into L0.
> 
>> If so, couldn’t we drop the HyperV vclock entirely and just
>> fold the adjustment into the core timekeeping data?  (Preferably the
>> actual core data, which would require core changes, but it could
>> plausibly be done in arch code, too.)
> 
> Not all Hyper-V hosts support reenlightenment notifications (and, if I'm
> not mistaken, you need to enable nesting for the VM to get the feature -
> and most VMs don't have this) so I think we'll have to keep Hyper-V
> vclock for the time being.
> 
> 

But this does suggest that the correct way to pass a clock through to an L2 guest where L0 is HV is to make L1 use the “tsc” clock and L2 use kvmclock (or something newer and better).  This would require adding support for atomic frequency changes all the way through the timekeeping and arch code.

John, tglx, would that be okay or crazy?