linux-kernel - Re: [PATCH] hyperv: root partition faults writing to VP ASSIST MSR PAGE

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Message-ID: <20210720162923.rsbl24v5lujbiddj@liuwe-devbox-debian-v2>
Date:   Tue, 20 Jul 2021 16:29:23 +0000
From:   Wei Liu <wei.liu@...nel.org>
To:     Michael Kelley <mikelley@...rosoft.com>
Cc:     Wei Liu <wei.liu@...nel.org>,
        Praveen Kumar <kumarpraveen@...ux.microsoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "viremana@...ux.microsoft.com" <viremana@...ux.microsoft.com>,
        Sunil Muthuswamy <sunilmut@...rosoft.com>,
        "nunodasneves@...ux.microsoft.com" <nunodasneves@...ux.microsoft.com>
Subject: Re: [PATCH] hyperv: root partition faults writing to VP ASSIST MSR
 PAGE

On Tue, Jul 20, 2021 at 04:20:44PM +0000, Michael Kelley wrote:
> From: Wei Liu <wei.liu@...nel.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