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From:   Michael Kelley <mikelley@microsoft.com>
To:     Wei Liu <wei.liu@kernel.org>,
        Praveen Kumar <kumarpraveen@linux.microsoft.com>
CC:     "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
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        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Dexuan Cui <decui@microsoft.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
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        "viremana@linux.microsoft.com" <viremana@linux.microsoft.com>,
        Sunil Muthuswamy <sunilmut@microsoft.com>,
        "nunodasneves@linux.microsoft.com" <nunodasneves@linux.microsoft.com>
Subject: RE: [PATCH] hyperv: root partition faults writing to VP ASSIST MSR
 PAGE
Thread-Topic: [PATCH] hyperv: root partition faults writing to VP ASSIST MSR
 PAGE
Thread-Index: AQHXfOKuCkOiUQ46GkaTOSxdT1kTAKtLuKKAgAAjIwCAAAKZAIAALFMg
Date:   Tue, 20 Jul 2021 16:20:44 +0000
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From: Wei Liu <wei.liu@kernel.org> Sent: Tuesday, July 20, 2021 6:35 AM
>=20
> On Tue, Jul 20, 2021 at 06:55:56PM +0530, Praveen Kumar wrote:
> [...]
> > >
> > >> +	if (hv_root_partition &&
> > >> +	    ms_hyperv.features & HV_MSR_APIC_ACCESS_AVAILABLE) {
> > >
> > > Is HV_MSR_APIC_ACCESS_AVAILABLE a root only flag? Shouldn't non-root
> > > kernel check this too?
> >
> > Yes, you are right. Will update this in v2. thanks.
>=20
> Please split adding this check to its own patch.
>=20
> Ideally one patch only does one thing.
>=20
> Wei.
>=20

I was just looking around in the Hyper-V TLFS, and I didn't see
anywhere that the ability to set up a VP Assist page is dependent
on HV_MSR_APIC_ACCESS_AVAILABLE.  Or did I just miss it?

Maybe the VP Assist page is not useful for the APIC EOI optimization
Purposes if !HV_MSR_APIC_ACCESS_AVAILABLE, but the VP Assist
page has other uses, such as for nested enlightenments.  So I
wonder if the VP Assist page setup really should be gated on
HV_MSR_APIC_ACCESS_AVAILABLE.

Michael