KVM: x86: Make nanoseconds per APIC bus cycle a VM variable

Introduce the VM variable "nanoseconds per APIC bus cycle" in
preparation to make the APIC bus frequency configurable.

The TDX architecture hard-codes the core crystal clock frequency to
25MHz and mandates exposing it via CPUID leaf 0x15. The TDX architecture
does not allow the VMM to override the value.

In addition, per Intel SDM:
    "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) divided by the value specified in
     the divide configuration register."

The resulting 25MHz APIC bus frequency conflicts with the KVM hardcoded
APIC bus frequency of 1GHz.

Introduce the VM variable "nanoseconds per APIC bus cycle" to prepare
for allowing userspace to tell KVM to use the frequency that TDX mandates
instead of the default 1Ghz. Doing so ensures that the guest doesn't have
a conflicting view of the APIC bus frequency.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
[reinette: rework changelog]
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/ae75ce37c6c38bb4efd10a0a41932984c40b24ac.1714081726.git.reinette.chatre@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/include/asm/kvm_host.h

@@ -1364,6 +1364,7 @@ struct kvm_arch {
 
 	u32 default_tsc_khz;
 	bool user_set_tsc;
+	u64 apic_bus_cycle_ns;
 
 	seqcount_raw_spinlock_t pvclock_sc;
 	bool use_master_clock;

arch/x86/kvm/hyperv.c

@@ -1737,7 +1737,8 @@ static int kvm_hv_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata,
 		data = (u64)vcpu->arch.virtual_tsc_khz * 1000;
 		break;
 	case HV_X64_MSR_APIC_FREQUENCY:
-		data = div64_u64(1000000000ULL, APIC_BUS_CYCLE_NS);
+		data = div64_u64(1000000000ULL,
+				 vcpu->kvm->arch.apic_bus_cycle_ns);
 		break;
 	default:
 		kvm_pr_unimpl_rdmsr(vcpu, msr);

arch/x86/kvm/lapic.c

@@ -1547,7 +1547,8 @@ static u32 apic_get_tmcct(struct kvm_lapic *apic)
 		remaining = 0;
 
 	ns = mod_64(ktime_to_ns(remaining), apic->lapic_timer.period);
-	return div64_u64(ns, (APIC_BUS_CYCLE_NS * apic->divide_count));
+	return div64_u64(ns, (apic->vcpu->kvm->arch.apic_bus_cycle_ns *
+			      apic->divide_count));
 }
 
 static void __report_tpr_access(struct kvm_lapic *apic, bool write)
@@ -1965,7 +1966,8 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
 
 static inline u64 tmict_to_ns(struct kvm_lapic *apic, u32 tmict)
 {
-	return (u64)tmict * APIC_BUS_CYCLE_NS * (u64)apic->divide_count;
+	return (u64)tmict * apic->vcpu->kvm->arch.apic_bus_cycle_ns *
+		(u64)apic->divide_count;
 }
 
 static void update_target_expiration(struct kvm_lapic *apic, uint32_t old_divisor)

arch/x86/kvm/lapic.h

@@ -16,7 +16,7 @@
 #define APIC_DEST_NOSHORT		0x0
 #define APIC_DEST_MASK			0x800
 
-#define APIC_BUS_CYCLE_NS       1
+#define APIC_BUS_CYCLE_NS_DEFAULT	1
 
 #define APIC_BROADCAST			0xFF
 #define X2APIC_BROADCAST		0xFFFFFFFFul

arch/x86/kvm/x86.c

@@ -12622,6 +12622,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
 
 	kvm->arch.default_tsc_khz = max_tsc_khz ? : tsc_khz;
+	kvm->arch.apic_bus_cycle_ns = APIC_BUS_CYCLE_NS_DEFAULT;
 	kvm->arch.guest_can_read_msr_platform_info = true;
 	kvm->arch.enable_pmu = enable_pmu;