linux-kernel - Re: [PATCH v3 3/7] x86/hyper-v: reenlightenment notifications support

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Date:   Tue, 16 Jan 2018 19:33:18 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
cc:     kvm@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Andy Lutomirski <luto@...nel.org>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        linux-kernel@...r.kernel.org, devel@...uxdriverproject.org
Subject: Re: [PATCH v3 3/7] x86/hyper-v: reenlightenment notifications
 support

On Tue, 16 Jan 2018, Vitaly Kuznetsov wrote:
> Hyper-V supports Live Migration notification. This is supposed to be used
> in conjunction with TSC emulation: when we are migrated to a host with
> different TSC frequency for some short period host emulates our accesses
> to TSC and sends us an interrupt to notify about the event. When we're
> done updating everything we can disable TSC emulation and everything will
> start working fast again.
> 
> We didn't need these notifications before as Hyper-V guests are not
> supposed to use TSC as a clocksource: in Linux we even mark it as unstable
> on boot. Guests normally use 'tsc page' clocksouce and host updates its
> values on migrations automatically.
> 
> Things change when we want to run nested virtualization: even when we pass
> through PV clocksources (kvm-clock or tsc page) to our guests we need to
> know TSC frequency and when it changes.
> 
> Hyper-V Top Level Functional Specification (as of v5.0b) wrongly specifies
> EAX:BIT(12) of CPUID:0x40000009 as the feature identification bit. The
> right one to check is EAX:BIT(13) of CPUID:0x40000003. I was assured that
> the fix in on the way.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>