linux-kernel - RE: [PATCH v3] hyperv: root partition faults writing to VP ASSIST MSR PAGE

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Message-ID: <MW4PR21MB2002964FBB69F00BBEE9F324C0E99@MW4PR21MB2002.namprd21.prod.outlook.com>
Date:   Tue, 27 Jul 2021 16:51:23 +0000
From:   Sunil Muthuswamy <sunilmut@...rosoft.com>
To:     Michael Kelley <mikelley@...rosoft.com>,
        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>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        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>,
        "nunodasneves@...ux.microsoft.com" <nunodasneves@...ux.microsoft.com>
Subject: RE: [PATCH v3] hyperv: root partition faults writing to VP ASSIST MSR
 PAGE

> >
> > -	if (*hvp) {
> > -		u64 val;
> > +	WARN_ON(!(*hvp));
> >
> > -		val = vmalloc_to_pfn(*hvp);
> > -		val = (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 = vmalloc_to_pfn(*hvp);
> >
> > -		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, val);
> > +		msr.enable = 1;
> > +		wrmsrl(HV_X64_MSR_VP_ASSIST_PAGE, msr.as_uint64);
> 
> 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 when
> 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 enable
bit needs to be toggled.

- Sunil