KVM: x86: Return the VM's configured APIC bus frequency when queried

When KVM_CAP_X86_APIC_BUS_CYCLES_NS is queried on a specific VM, return the
VM's configured APIC bus frequency, not KVM's default.  Aside from the fact
that returning the default frequency is blatantly wrong if userspace has
changed the frequency, returning the configured frequency means userspace
can blindly trust the result, e.g. when filling PV CPUID information that
communicates the APIC bus frequency to the guest.

Fixes: 6fef518 ("KVM: x86: Add a capability to configure bus frequency for APIC timer")
Reported-by: David Woodhouse <dwmw2@infradead.org>
Closes: https://lore.kernel.org/all/ab84153e33fbe7c25667f595c56b310d4d5a93ef.camel@infradead.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260522173526.3539407-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -4876,7 +4876,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		r = tdp_enabled;
 		break;
 	case KVM_CAP_X86_APIC_BUS_CYCLES_NS:
-		r = APIC_BUS_CYCLE_NS_DEFAULT;
+		r = kvm ? kvm->arch.apic_bus_cycle_ns : APIC_BUS_CYCLE_NS_DEFAULT;
 		break;
 	case KVM_CAP_EXIT_HYPERCALL:
 		r = KVM_EXIT_HYPERCALL_VALID_MASK;