KVM: x86: do not go through vcpu in __get_kvmclock_ns

Going through the first VCPU is wrong if you follow a KVM_SET_CLOCK with
a KVM_GET_CLOCK immediately after, without letting the VCPU run and
call kvm_guest_time_update.

To fix this, compute the kvmclock value ourselves, using the master
clock (tsc, nsec) pair as the base and the host CPU frequency as
the scale.

Reported-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3017de0..7d3d9d4 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1724,18 +1724,23 @@ static void kvm_gen_update_masterclock(struct kvm *kvm)
 
 static u64 __get_kvmclock_ns(struct kvm *kvm)
 {
-       struct kvm_vcpu *vcpu = kvm_get_vcpu(kvm, 0);
        struct kvm_arch *ka = &kvm->arch;
-       s64 ns;
+       struct pvclock_vcpu_time_info hv_clock;
 
-       if (vcpu->arch.hv_clock.flags & PVCLOCK_TSC_STABLE_BIT) {
-               u64 tsc = kvm_read_l1_tsc(vcpu, rdtsc());
-               ns = __pvclock_read_cycles(&vcpu->arch.hv_clock, tsc);
-       } else {
-               ns = ktime_get_boot_ns() + ka->kvmclock_offset;
+       spin_lock(&ka->pvclock_gtod_sync_lock);
+       if (!ka->use_master_clock) {
+               spin_unlock(&ka->pvclock_gtod_sync_lock);
+               return ktime_get_boot_ns() + ka->kvmclock_offset;
        }
 
-       return ns;
+       hv_clock.tsc_timestamp = ka->master_cycle_now;
+       hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
+       spin_unlock(&ka->pvclock_gtod_sync_lock);
+
+       kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
+                          &hv_clock.tsc_shift,
+                          &hv_clock.tsc_to_system_mul);
+       return __pvclock_read_cycles(&hv_clock, rdtsc());
 }
 
 u64 get_kvmclock_ns(struct kvm *kvm)