Re: [PATCH] kvm: x86: emulate MSR_PLATFORM_INFO msr bits

[Sean Christopherson]

On Wed, Aug 23, 2023, Xiaoyao Li wrote:
> On 8/22/2023 12:11 AM, Sean Christopherson wrote:
> > > Set these msr bits (needed by turbostat on intel platform) in KVM by
> > > default.  Of cource, QEMU can also set MSR value by need. It does not
> > > conflict.
> > 
> > It doesn't conflict per se, but it's still problematic.  By stuffing a default
> > value, KVM _forces_ userspace to override the MSR to align with the topology and
> > CPUID defined by userspace.
> 
> I don't understand how this MSR is related to topology and CPUID?

Heh, looked at the SDM to double check myself, and the first hit when searching
for MSR_PLATFORM_INFO says:

  When TSC scaling is enabled for a guest using Intel PT, the VMM should ensure
  that the value of Maximum Non-Turbo Ratio[15:8] in MSR_PLATFORM_INFO (MSR 0CEH)
  and the TSC/”core crystal clock” ratio (EBX/EAX) in CPUID leaf 15H are set in
  a manner consistent with the resulting TSC rate that will be visible to the VM.

As Chao pointed out, the MSR is technically per package, so a weird setup could
have sockets with different frequencies, or enumerate a virtual topology to the
guest with such a configuration.  I doubt/hope no one actually does something
like that, but it's theoretically possible, and one of the many reasons why KVM
needs to stay out of the way and let userspace define the vCPU model.

> > And if userspace uses KVM's "default" CPUID, or lack thereof, using the
> > underlying values from hardware are all but guaranteed to be wrong.
> 
> Could you please elaborate?

I guess an empty CPUID would probably be ok?  If there's no CPUID.0x15, it can't
be wrong.  It's largely a moot point though, I highly doubt anyone runs a "real"
VM without populating _something_ in guest CPUID.