Sean Christopherson <seanjc@xxxxxxxxxx> writes: > On Tue, Feb 04, 2025, Nikunj A Dadhania wrote: >> Sean Christopherson <seanjc@xxxxxxxxxx> writes: >> >> > When running as a TDX guest, explicitly override the TSC frequency >> > calibration routine with CPUID-based calibration instead of potentially >> > relying on a hypervisor-controlled PV routine. For TDX guests, CPUID.0x15 >> > is always emulated by the TDX-Module, i.e. the information from CPUID is >> > more trustworthy than the information provided by the hypervisor. >> > >> > To maintain backwards compatibility with TDX guest kernels that use native >> > calibration, and because it's the least awful option, retain >> > native_calibrate_tsc()'s stuffing of the local APIC bus period using the >> > core crystal frequency. While it's entirely possible for the hypervisor >> > to emulate the APIC timer at a different frequency than the core crystal >> > frequency, the commonly accepted interpretation of Intel's SDM is that APIC >> > timer runs at the core crystal frequency when that latter is enumerated via >> > CPUID: >> > >> > The APIC timer frequency will be the processor’s bus clock or core >> > crystal clock frequency (when TSC/core crystal clock ratio is enumerated >> > in CPUID leaf 0x15). >> > >> > If the hypervisor is malicious and deliberately runs the APIC timer at the >> > wrong frequency, nothing would stop the hypervisor from modifying the >> > frequency at any time, i.e. attempting to manually calibrate the frequency >> > out of paranoia would be futile. >> > >> > Deliberately leave the CPU frequency calibration routine as is, since the >> > TDX-Module doesn't provide any guarantees with respect to CPUID.0x16. >> >> Does TDX use kvmclock? > > A TDX guest can. That's up to the host (expose kvmclock) and the guest (enable > kvmclock). > >> If yes, kvmclock would have registered the CPU frequency calibration routine: >> >> tsc_register_calibration_routines(kvm_get_tsc_khz, kvm_get_cpu_khz, >> tsc_properties); >> >> so TDX will use kvm_get_cpu_khz(), which will either use CPUID.0x16 or >> PV clock, is this on the expected line ? > > What do you mean by "is this on the expected line"? If you are asking "is this > intended", Yes, that is what I meant. > then the answer is "yes, working as intended". As above, the TDX-Module > doesn't emulate CPUID.0x16, so no matter what, the guest is relying on the untrusted > hypervisor to get the CPU frequency. If someone thinks that TDX guests should > assume the CPU runs as the same frequency as the TSC, a la SNP's Secure TSC, then > they are welcome to propose such a change. Ok, that makes sense. Regards Nikunj
