MSR accesses that slows down the hypervisor/host

[Wei Hu]

> > > From: Konstantin Belousov <kostikbel at gmail.com>
> > > Sent: Wednesday, September 16, 2020 9:57 PM
> > > To: Wei Hu <weh at microsoft.com>
> > > Cc: freebsd-hackers at freebsd.org
> > > Subject: Re: MSR accesses that slows down the hypervisor/host
> > >
> > > Where do you see accesses to MSR_LS_CFG ?  I can only find
> > > manipulations of that MSR in init_amd(), and then it is all under
> > > check that we are not virtualized.
> > >
> > Yes, it is only accessed in init_amd() at boot time. So it is less
> > concerned. MSR_AMDK8_IPM is accessed in cpu_idle() all the time, so it is
> the key place to optimize.
> >
> > > For MSR_AMDK8_IPM access in cpu_idle(), it seems that the workaround
> > > was applied too wide. It might be that we do not need to do it on
> > > recent CPUs, but I need to spent more time looking at datasheets to
> confirm/deny.
> > >
> > > But, do you (hypervisor) indeed allow guest to initiate C1 or deeper idle
> state ?
> > > If not, perhaps as the first measure, we can avoid manipulating
> > > MSR_AMDK8_IPM under hypervisor at all.
> >
> > You are right a guest cannot initiate C1 or deeper idle state when running on
> Hyper-V.
> > So skipping the read of MSR_AMDK8_IPM when running under this
> > hypervisor would Be a viable solution.
> 
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Thanks for the quick response. I will review the change and do some tests on Hyper-V.

Wei