Re: [PATCH v2 0/3] Support "generic" CPUID timing leaf as KVM guest and host

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 21, 2025, David Woodhouse wrote:
> On Thu, 2025-08-21 at 09:26 -0700, Sean Christopherson wrote:
> > On Sat, Aug 16, 2025, David Woodhouse wrote:
> > > In https://lkml.org/lkml/2008/10/1/246 VMware proposed a generic standard
> > > for harmonising CPUID between hypervisors. It was mostly shot down in
> > > flames, but the generic timing leaf at 0x4000_0010 didn't quite die.
> > > 
> > > Mostly the hypervisor leaves at 0x4000_0xxx are very hypervisor-specific,
> > > but XNU and FreeBSD as guests will look for 0x4000_0010 unconditionally,
> > > under any hypervisor. The EC2 Nitro hypervisor has also exposed TSC
> > > frequency information in this leaf, since 2020.
> > > 
> > > As things stand, KVM guests have to reverse-calculate the TSC frequency
> > > from the mul/shift information given to them in the KVM clock to convert
> > > ticks into nanoseconds, with a corresponding loss of precision.
> > 
> > I would rather have the VMM use the Intel-define CPUID.0x15 to enumerate the
> > TSC frequency. 
> 
> The problem with that is that it's been quite unreliable. The kernel
> doesn't trust it even on chips as recent (hah) as Skylake. I'd be
> happier to trust what the hypervisor explicitly gives us. But yes, it
> should be *one* of the sources of information before we reverse-
> calculate it from the pvclock. 

Sorry, by "the VMM use" I mean have the host, e.g. QEMU, explicitly define TSC
frequency in CPUID.0x15 and CPU frequency in CPUID.0x16.  And then on the
KVM-as-a-guest side of things, trust those leaves when they're available.

So same idea as having the VMM fill 0x4000_0010, but piggyback the Intel-defined
leaves instead of the VMware-defined leaf.  One of the reasons I'd like to go
that route is to avoid having to choose one or the other when running under TDX,
where CPUID.{0x15,0x16} are provided by the "trusted" TDX-Module, but any PV
leaf is not.

Dunno how feasible it is to get non-Linux guests on board though...