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Date:   Tue, 20 Jul 2021 16:29:23 +0000
From:   Wei Liu <wei.liu@kernel.org>
To:     Michael Kelley <mikelley@microsoft.com>
Cc:     Wei Liu <wei.liu@kernel.org>,
        Praveen Kumar <kumarpraveen@linux.microsoft.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        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>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>, "x86@kernel.org" <x86@kernel.org>,
        "hpa@zytor.com" <hpa@zytor.com>,
        "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
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On Tue, Jul 20, 2021 at 04:20:44PM +0000, Michael Kelley wrote:
> From: Wei Liu <wei.liu@kernel.org> Sent: Tuesday, July 20, 2021 6:35 AM
> > 
> > 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.
> > 
> > Please split adding this check to its own patch.
> > 
> > Ideally one patch only does one thing.
> > 
> > Wei.
> > 
> 
> 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?

The feature bit Praveen used is wrong and should be fixed.

Per internal discussion this is gated by the AccessIntrCtrlRegs bit.

Wei.

> 
> 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