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From:   Sunil Muthuswamy <sunilmut@microsoft.com>
To:     Michael Kelley <mikelley@microsoft.com>,
        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>
CC:     KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "wei.liu@kernel.org" <wei.liu@kernel.org>,
        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>,
        "nunodasneves@linux.microsoft.com" <nunodasneves@linux.microsoft.com>
Subject: RE: [PATCH v3] hyperv: root partition faults writing to VP ASSIST MSR
 PAGE
Thread-Topic: [PATCH v3] hyperv: root partition faults writing to VP ASSIST
 MSR PAGE
Thread-Index: AQHXgtPz/n7CxiZ6zUi9GbDbsi36lqtXBSEAgAADfKA=
Date:   Tue, 27 Jul 2021 16:51:23 +0000
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> >
> > -	if (*hvp) {
> > -		u64 val;
> > +	WARN_ON(!(*hvp));
> >
> > -		val =3D vmalloc_to_pfn(*hvp);
> > -		val =3D (val << HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT) |
> > -			HV_X64_MSR_VP_ASSIST_PAGE_ENABLE;
> > +	if (*hvp) {
> > +		if (!hv_root_partition)
> > +			msr.guest_physical_address =3D vmalloc_to_pfn(*hvp);
> >
> > -		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, val);
> > +		msr.enable =3D 1;
> > +		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
>=20
> This version has a substantive difference compared with previous versions
> in that the "enable" bit is being set and written back to the MSR even wh=
en
> running in the root partition.  Is that intentional?

Yes, the hypervisor won't allow the root to change the page address. So, to
enable/disable it, the page address needs to be preserved and just the enab=
le
bit needs to be toggled.

- Sunil