VCPU TSC is not cleared by a warm reset (*), which leaves many Linux guests vulnerable to the overflow in cyc2ns_offset fixed by upstream commit 9993bc635d01a6ee7f6b833b4ee65ce7c06350b1 ("sched/x86: Fix overflow in cyc2ns_offset").
To put it in a nutshell, if a Linux guest without the patch above applied has been up more than 208 days and attempts a warm reset chances are that the newly booted kernel will panic or hang. (*) Intel Xeon E5 processors show the same broken behavior due to the errata "TSC is Not Affected by Warm Reset" (Intel® Xeon® Processor E5 Family Specification Update - August 2013): "The TSC (Time Stamp Counter MSR 10H) should be cleared on reset. Due to this erratum the TSC is not affected by warm reset." Cc: sta...@vger.kernel.org Cc: Will Auld <will.a...@intel.com> Cc: Marcelo Tosatti <mtosa...@redhat.com> Signed-off-by: Fernando Luis Vazquez Cao <ferna...@oss.ntt.co.jp> --- --- qemu-orig/target-i386/kvm.c 2013-11-28 07:02:45.000000000 +0900 +++ qemu/target-i386/kvm.c 2013-12-05 14:47:03.085738175 +0900 @@ -1125,6 +1125,8 @@ static int kvm_put_msrs(X86CPU *cpu, int kvm_msr_entry_set(&msrs[n++], MSR_VM_HSAVE_PA, env->vm_hsave); } if (has_msr_tsc_adjust) { + if (level == KVM_PUT_RESET_STATE) + env->tsc_adjust = 0; kvm_msr_entry_set(&msrs[n++], MSR_TSC_ADJUST, env->tsc_adjust); } if (has_msr_misc_enable) { @@ -1139,22 +1141,22 @@ static int kvm_put_msrs(X86CPU *cpu, int kvm_msr_entry_set(&msrs[n++], MSR_LSTAR, env->lstar); } #endif - if (level == KVM_PUT_FULL_STATE) { + /* + * The following MSRs have side effects on the guest or are too heavy + * for normal writeback. Limit them to reset or full state updates. + */ + if (level >= KVM_PUT_RESET_STATE) { + if (level == KVM_PUT_RESET_STATE) + env->tsc = 0; /* * 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) { + if (smp_cpus == 1 || env->tsc != 0 || level == KVM_PUT_RESET_STATE) { kvm_msr_entry_set(&msrs[n++], MSR_IA32_TSC, env->tsc); } - } - /* - * The following MSRs have side effects on the guest or are too heavy - * for normal writeback. Limit them to reset or full state updates. - */ - if (level >= KVM_PUT_RESET_STATE) { kvm_msr_entry_set(&msrs[n++], MSR_KVM_SYSTEM_TIME, env->system_time_msr); kvm_msr_entry_set(&msrs[n++], MSR_KVM_WALL_CLOCK, env->wall_clock_msr); -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html