From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx> Add KVM_CAP_X86_BUS_FREQUENCY_CONTROL capability to configure the core crystal clock (or processor's bus clock) for APIC timer emulation. Allow KVM_ENABLE_CAPABILITY(KVM_CAP_X86_BUS_FREUQNCY_CONTROL) to set the frequency. When using this capability, the user space VMM should configure CPUID[0x15] to advertise the frequency. TDX virtualizes CPUID[0x15] for the core crystal clock to be 25MHz. The x86 KVM hardcodes its freuqncy for APIC timer to be 1GHz. This mismatch causes the vAPIC timer to fire earlier than the guest expects. [1] The KVM APIC timer emulation uses hrtimer, whose unit is nanosecond. There are options to reconcile the mismatch. 1) Make apic bus clock frequency configurable (this patch). 2) TDX KVM code adjusts TMICT value. This is hacky and it results in losing MSB bits from 32 bit width to 30 bit width. 3). Make the guest kernel use tsc deadline timer instead of acpi oneshot/periodic timer. This is guest kernel choice. It's out of control of VMM. [1] https://lore.kernel.org/lkml/20231006011255.4163884-1-vannapurve@xxxxxxxxxx/ Isaku Yamahata (2): KVM: x86: Make the hardcoded APIC bus frequency vm variable KVM: X86: Add a capability to configure bus frequency for APIC timer arch/x86/include/asm/kvm_host.h | 2 ++ arch/x86/kvm/hyperv.c | 2 +- arch/x86/kvm/lapic.c | 6 ++++-- arch/x86/kvm/lapic.h | 4 ++-- arch/x86/kvm/x86.c | 16 ++++++++++++++++ include/uapi/linux/kvm.h | 1 + 6 files changed, 26 insertions(+), 5 deletions(-) -- 2.25.1
