KVM: x86: workaround SuSE's 2.6.16 pvclock vs masterclock issue

SuSE's 2.6.16 kernel fails to boot if the delta between tsc_timestamp
and rdtsc is larger than a given threshold:

 * If we get more than the below threshold into the future, we rerequest
 * the real time from the host again which has only little offset then
 * that we need to adjust using the TSC.
 *
 * For now that threshold is 1/5th of a jiffie. That should be good
 * enough accuracy for completely broken systems, but also give us swing
 * to not call out to the host all the time.
 */
#define PVCLOCK_DELTA_MAX ((1000000000ULL / HZ) / 5)

Disable masterclock support (which increases said delta) in case the
boot vcpu does not use MSR_KVM_SYSTEM_TIME_NEW.

Upstreams kernels which support pvclock vsyscalls (and therefore make
use of PVCLOCK_STABLE_BIT) use MSR_KVM_SYSTEM_TIME_NEW.

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

1 files changed, 14 insertions, 1 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index dcb996bfafa4..917672f8034a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1542,7 +1542,8 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
                                         &ka->master_cycle_now);
 
         ka->use_master_clock = host_tsc_clocksource && vcpus_matched
-                                && !backwards_tsc_observed;
+                                && !backwards_tsc_observed
+                                && !ka->boot_vcpu_runs_old_kvmclock;
 
         if (ka->use_master_clock)
                 atomic_set(&kvm_guest_has_master_clock, 1);
@@ -2174,8 +2175,20 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
         case MSR_KVM_SYSTEM_TIME_NEW:
         case MSR_KVM_SYSTEM_TIME: {
                 u64 gpa_offset;
+                struct kvm_arch *ka = &vcpu->kvm->arch;
+
                 kvmclock_reset(vcpu);
 
+                if (vcpu->vcpu_id == 0 && !msr_info->host_initiated) {
+                        bool tmp = (msr == MSR_KVM_SYSTEM_TIME);
+
+                        if (ka->boot_vcpu_runs_old_kvmclock != tmp)
+                                set_bit(KVM_REQ_MASTERCLOCK_UPDATE,
+                                        &vcpu->requests);
+
+                        ka->boot_vcpu_runs_old_kvmclock = tmp;
+                }
+
                 vcpu->arch.time = data;
                 kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);