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

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Date:   Wed, 03 Oct 2018 14:01:38 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
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

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.

-- 
Vitaly