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Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <87murvdysd.fsf@vitty.brq.redhat.com>
Date:   Wed, 3 Oct 2018 07:20:48 -0700
Cc:     Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        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>
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References: <20180914125006.349747096@linutronix.de> <CALCETrU+jP2hPLoJWTYJKTvr7-=YLwtk1=cZ_uizHYQZ1z4P-w@mail.gmail.com> <87sh1ne64t.fsf@vitty.brq.redhat.com> <4B6A97E1-17E6-40F2-A7A0-87731668A07C@amacapital.net> <87murvdysd.fsf@vitty.brq.redhat.com>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Oct 3, 2018, at 5:01 AM, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>=20
> Andy Lutomirski <luto@amacapital.net> writes:
>=20
>>> On Oct 3, 2018, at 2:22 AM, Vitaly Kuznetsov <vkuznets@redhat.com> wrote=
:
>>>=20
>>> Andy Lutomirski <luto@kernel.org> writes:
>>>=20
>>>> Hi Vitaly, Paolo, Radim, etc.,
>>>>=20
>>> 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.
>>=20
>> That=E2=80=99s delightful!  Does the emulation magic also work for L1 use=
r
>> mode?
>=20
> As far as I understand - yes, all rdtsc* calls will trap into L0.
>=20
>> If so, couldn=E2=80=99t 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.)
>=20
> 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.
>=20
>=20

But this does suggest that the correct way to pass a clock through to an L2 g=
uest where L0 is HV is to make L1 use the =E2=80=9Ctsc=E2=80=9D clock and L2=
 use kvmclock (or something newer and better).  This would require adding su=
pport for atomic frequency changes all the way through the timekeeping and a=
rch code.

John, tglx, would that be okay or crazy?=