[v3,07/10] qemu-kvm: Cleanup/fix TSC and PV clock writeback

Commit Message

Jan Kiszka Feb. 25, 2010, 8:48 a.m. UTC

None

Patch

diff --git a/qemu-kvm-x86.c b/qemu-kvm-x86.c
index 84fd7fa..285c05a 100644
--- a/qemu-kvm-x86.c
+++ b/qemu-kvm-x86.c
@@ -966,7 +966,15 @@  void kvm_arch_load_regs(CPUState *env, int level)
     }
 #endif
     if (level == KVM_PUT_FULL_STATE) {
-        set_msr_entry(&msrs[n++], MSR_IA32_TSC, env->tsc);
+        /*
+         * KVM is yet unable to synchronize TSC values of multiple VCPUs on
+         * writeback. Until this is fixed, we only write the offset to SMP
+         * guests after migration, desynchronizing the VCPUs, but avoiding
+         * huge jump-backs that would occur without any writeback at all.
+         */
+        if (smp_cpus == 1 || env->tsc != 0) {
+            set_msr_entry(&msrs[n++], MSR_IA32_TSC, env->tsc);
+        }
         set_msr_entry(&msrs[n++], MSR_KVM_SYSTEM_TIME, env->system_time_msr);
         set_msr_entry(&msrs[n++], MSR_KVM_WALL_CLOCK, env->wall_clock_msr);
     }