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...
