linux-kernel - Re: [PATCH 3/6] x86/hyper-v: reenlightenment notifications support

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Date:   Mon, 11 Dec 2017 20:10:24 +0300
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>, linux-kernel@...r.kernel.org,
        devel@...uxdriverproject.org
Subject: Re: [PATCH 3/6] x86/hyper-v: reenlightenment notifications support

On Mon, Dec 11, 2017 at 10:56:33AM +0100, Vitaly Kuznetsov wrote:
> Roman Kagan <rkagan@...tuozzo.com> writes:
> > On Fri, Dec 08, 2017 at 11:49:57AM +0100, Vitaly Kuznetsov wrote:
> >> +void register_hv_tsc_update(void (*cb)(void))
> >> +{
> >
> > The function name seems unfortunate.  IMHO such a name suggests
> > registering a callback on a notifier chain (rather than unconditionally
> > replacing the old callback), and having no other side effects.
> 
> I see, any suggestion? register_hv_reenlightenment_cb? register_hv_tscchange_cb?

IMHO arm_hv_reenlightenment_cb or arm_hv_tscchange_cb would be better,
but I'm not very good at giving descriptive names.

> 
> >
> >> +	struct hv_reenlightenment_control re_ctrl = {
> >> +		.vector = HYPERV_REENLIGHTENMENT_VECTOR,
> >> +		.enabled = 1,
> >> +		.target_vp = hv_vp_index[smp_processor_id()]
> >> +	};
> >> +	struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};
> >> +
> >> +	if (!(ms_hyperv.features & HV_X64_ACCESS_REENLIGHTENMENT))
> >> +		return;
> >
> > What happens then?  L2 guests keep running with their clocks ticking at
> > a different speed?
> >
> 
> In reallity this never happens -- in case nested virtualization is
> supported reenlightenment is also available. In theory, L0 can emulate
> TSC acceess for forever after migration.

I would think that Hyper-V only started rdtsc emulation if
TSC_EMULATION_CONTROL was turned on, which wouldn't happen here.

But indeed, normally this shouldn't be a problem.  It may make sense
just to issue a warning if the feature is unsupported, though.

Roman.