Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Andy Lutomirski <luto@amacapital.net>
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>
Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support
In-Reply-To: <4B6A97E1-17E6-40F2-A7A0-87731668A07C@amacapital.net>
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>
Date:   Wed, 03 Oct 2018 14:01:38 +0200
Message-ID: <87murvdysd.fsf@vitty.brq.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Andy Lutomirski <luto@amacapital.net> writes:

>> On Oct 3, 2018, at 2:22 AM, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>> 
>> Andy Lutomirski <luto@kernel.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