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

[Michael Kelley]

From: Wei Liu <wei.liu@xxxxxxxxxx> 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?

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