Re: [PATCH] [RFC] kvm: x86: emulate APERF/MPERF registers

[Jim Mattson]

On Fri, Apr 24, 2020 at 7:46 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Fri, Apr 24, 2020 at 12:01:43PM +0200, Peter Zijlstra wrote:
> > On Fri, Apr 24, 2020 at 01:08:55PM +0800, Li RongQing wrote:

> This requires four RDMSRs per VMX transition.  Doing that unconditionally
> will drastically impact performance.  Not to mention that reading the MSRs
> without checking for host support will generate #GPs and WARNs on hardware
> without APERFMPERF.
>
> Assuming we're going forward with this, at an absolute minimum the RDMSRs
> need to be wrapped with checks on host _and_ guest support for the emulated
> behavior.  Given the significant overhead, this might even be something
> that should require an extra opt-in from userspace to enable.

I would like to see performance data before enabling this unconditionally.