Re: KVM: x86: disable master clock if TSC is reset during suspend

[Paolo Bonzini <pbonzini@redhat.com>]

Il 14/05/2014 09:26, Marcelo Tosatti ha scritto:
>
> Updating system_time from the kernel clock once master clock
> has been enabled can result in time backwards event, in case
> kernel clock frequency is lower than TSC frequency.
>
> Disable master clock in case its necessary to update it
> from the resume path.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index de0931c..9f5dd40 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -106,6 +106,8 @@ EXPORT_SYMBOL_GPL(kvm_max_guest_tsc_khz);
>  static u32 tsc_tolerance_ppm = 250;
>  module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
>
> +static bool suspend_backwards_tsc = false;
> +
>  #define KVM_NR_SHARED_MSRS 16
>
>  struct kvm_shared_msrs_global {
> @@ -1472,6 +1474,8 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
>  					&ka->master_cycle_now);
>
>  	ka->use_master_clock = host_tsc_clocksource & vcpus_matched;
> +	if (suspend_backwards_tsc)
> +		ka->use_master_clock = 0;

Use an && instead of an "if"?  (Also, why "&" in the other assignment of 
ka->use_master_clock)?

>  	if (ka->use_master_clock)
>  		atomic_set(&kvm_guest_has_master_clock, 1);
> @@ -6939,6 +6943,7 @@ int kvm_arch_hardware_enable(void *garbage)
>  	 */
>  	if (backwards_tsc) {
>  		u64 delta_cyc = max_tsc - local_tsc;
> +		suspend_backwards_tsc = true;
>  		list_for_each_entry(kvm, &vm_list, vm_list) {
>  			kvm_for_each_vcpu(i, vcpu, kvm) {
>  				vcpu->arch.tsc_offset_adjustment += delta_cyc;
>

Why a global and not a variable inside kvm?  The same question holds for 
kvm_guest_has_master_clock.

Paolo