Re: [PATCH] make tsc stable over migration and machine start

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-02-01 20:17, Glauber Costa wrote:
> If the machine is stopped, we should not record two different tsc values
> upon a save operation. The same problem happens with kvmclock.
> 
> But kvmclock is taking a different diretion, being now seen as a separate
> device. Since this is unlikely to happen with the tsc, I am taking the
> approach here of simply registering a handler for state change, and
> using a per-CPUState variable that prevents double updates for the TSC.
> 
> Signed-off-by: Glauber Costa <glommer@redhat.com>
> ---
>  target-i386/cpu.h |    1 +
>  target-i386/kvm.c |   19 ++++++++++++++++++-
>  2 files changed, 19 insertions(+), 1 deletions(-)
> 
> diff --git a/target-i386/cpu.h b/target-i386/cpu.h
> index 6d619e8..7f1c4f8 100644
> --- a/target-i386/cpu.h
> +++ b/target-i386/cpu.h
> @@ -732,6 +732,7 @@ typedef struct CPUX86State {
>      uint32_t sipi_vector;
>      uint32_t cpuid_kvm_features;
>      uint32_t cpuid_svm_features;
> +    uint8_t  update_tsc;

bool please.

>      
>      /* in order to simplify APIC support, we leave this pointer to the
>         user */
> diff --git a/target-i386/kvm.c b/target-i386/kvm.c
> index ecb8405..c3925be 100644
> --- a/target-i386/kvm.c
> +++ b/target-i386/kvm.c
> @@ -302,6 +302,16 @@ void kvm_inject_x86_mce(CPUState *cenv, int bank, uint64_t status,
>  
>  static int _kvm_arch_init_vcpu(CPUState *env);
>  
> +static void cpu_update_state(void *opaque, int running, int reason)
> +{
> +    CPUState *env = opaque;
> +
> +    if (!running) {
> +        env->update_tsc = 1;
> +    }
> +}
> +
> +

Additional blank line.

>  int kvm_arch_init_vcpu(CPUState *env)
>  {
>      int r;
> @@ -444,6 +454,8 @@ int kvm_arch_init_vcpu(CPUState *env)
>      }
>  #endif
>  
> +    qemu_add_vm_change_state_handler(cpu_update_state, env);
> +
>      return kvm_vcpu_ioctl(env, KVM_SET_CPUID2, &cpuid_data);
>  }
>  
> @@ -1093,7 +1105,12 @@ static int kvm_get_msrs(CPUState *env)
>  	msrs[n++].index = MSR_STAR;
>      if (kvm_has_msr_hsave_pa(env))
>          msrs[n++].index = MSR_VM_HSAVE_PA;
> -    msrs[n++].index = MSR_IA32_TSC;
> +
> +    if (env->update_tsc) {
> +        msrs[n++].index = MSR_IA32_TSC;
> +        env->update_tsc = 0;
> +    }
> +
>  #ifdef TARGET_X86_64
>      if (lm_capable_kernel) {
>          msrs[n++].index = MSR_CSTAR;

Not quite the logic I'm using for kvmclock:

cpu_update_state()
	if running
		tsc_valid = false;

kvm_get_msrs()
	...
	if (!tsc_valid)
		read_tsc
		tsc_valid = !vm_running;

That ensure we always read the tsc while the VM is running, and not only
after it was stopped (might otherwise be "surprising" when once
visualizing the MSRs).

Jan


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