RE: [PATCH] hyperv: root partition faults writing to VP ASSIST MSR PAGE

From: Michael Kelley <mikelley@microsoft.com>
To: Wei Liu <wei.liu@kernel.org>
Cc: 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 PAGE
Date: Wed, 21 Jul 2021 04:10:33 +0000	[thread overview]
Message-ID: <MWHPR21MB159302588AD32CA605192398D7E39@MWHPR21MB1593.namprd21.prod.outlook.com> (raw)
In-Reply-To: <20210720162923.rsbl24v5lujbiddj@liuwe-devbox-debian-v2>

From: Wei Liu <wei.liu@kernel.org> Sent: Tuesday, July 20, 2021 9:29 AM
> 
> 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.
> 

The AccessIntrCtrlRegs bit *is* HV_MSR_APIC_ACCESS_AVAILABLE.
Both are defined as bit 4 of the Partition Privilege flags.  :-)   I don't
know why the names don't line up.   Even so, it's not clear to me that
AccessIntrCtrlRegs has any bearing on the VP Assist page.  I see this
description of AccessIntrCtrlRegs:

The partition has access to the synthetic MSRs associated with the
APIC (HV_X64_MSR_EOI, HV_X64_MSR_ICR and HV_X64_MSR_TPR).
If this flag is cleared, accesses to these MSRs results in a #GP fault if
the MSR intercept is not installed.

But maybe you have additional info that applies to the root
partition that is not in the TLFS.

Michael