KVM: x86: require matched TSC offsets for master clock

With master clock, a pvclock clock read calculates:

ret = system_timestamp + [ (rdtsc + tsc_offset) - tsc_timestamp ]

Where 'rdtsc' is the host TSC.

system_timestamp and tsc_timestamp are unique, one tuple
per VM: the "master clock".

Given a host with synchronized TSCs, its obvious that
guest TSC must be matched for the above to guarantee monotonicity.

Allow master clock usage only if guest TSCs are synchronized.

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

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 32f0e4a063b7..9fb6d8da7a43 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -561,6 +561,7 @@ struct kvm_arch {
         u64 cur_tsc_write;
         u64 cur_tsc_offset;
         u8  cur_tsc_generation;
+        int nr_vcpus_matched_tsc;
 
         spinlock_t pvclock_gtod_sync_lock;
         bool use_master_clock;