Re: [PATCH] kvmclock: clock should count only if vm is running

[Paolo Bonzini <pbonzini@redhat.com>]

Hi Marcelo, sorry for the late review.

Il 08/06/2013 04:00, Marcelo Tosatti ha scritto:
> kvmclock should not count while vm is paused, because:
> 
> 1) if the vm is paused for long periods, timekeeping 
> math can overflow while converting the (large) clocksource 
> delta to nanoseconds.
> 
> 2) Users rely on CLOCK_MONOTONIC to count run time, that is, 
> time which OS has been in a runnable state (see CLOCK_BOOTTIME).

Do you have any ideas on how to implement CLOCK_BOOTTIME for kvmclock?
I think we need to add more fields for the delta between CLOCK_MONOTONIC
and CLOCK_BOOTTIME.

> Change kvmclock driver so as to save clock value when vm transitions
> from runnable to stopped state, and to restore clock value from stopped
> to runnable transition.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
> index 87d4d0f..7d2d005 100644
> --- a/hw/i386/kvm/clock.c
> +++ b/hw/i386/kvm/clock.c
> @@ -28,38 +28,6 @@ typedef struct KVMClockState {
>      bool clock_valid;
>  } KVMClockState;
>  
> -static void kvmclock_pre_save(void *opaque)
> -{
> -    KVMClockState *s = opaque;
> -    struct kvm_clock_data data;
> -    int ret;
> -
> -    if (s->clock_valid) {
> -        return;
> -    }
> -    ret = kvm_vm_ioctl(kvm_state, KVM_GET_CLOCK, &data);
> -    if (ret < 0) {
> -        fprintf(stderr, "KVM_GET_CLOCK failed: %s\n", strerror(ret));
> -        data.clock = 0;
> -    }
> -    s->clock = data.clock;
> -    /*
> -     * If the VM is stopped, declare the clock state valid to avoid re-reading
> -     * it on next vmsave (which would return a different value). Will be reset
> -     * when the VM is continued.
> -     */
> -    s->clock_valid = !runstate_is_running();
> -}
> -
> -static int kvmclock_post_load(void *opaque, int version_id)
> -{
> -    KVMClockState *s = opaque;
> -    struct kvm_clock_data data;
> -
> -    data.clock = s->clock;
> -    data.flags = 0;
> -    return kvm_vm_ioctl(kvm_state, KVM_SET_CLOCK, &data);
> -}
>  
>  static void kvmclock_vm_state_change(void *opaque, int running,
>                                       RunState state)
> @@ -70,8 +38,18 @@ static void kvmclock_vm_state_change(void *opaque, int running,
>      int ret;
>  
>      if (running) {
> +        struct kvm_clock_data data;
> +

Do we need an "if (!s->clock_valid) return;" here, or an assertion?  (Or
alternatively, what happens if s->clock_valid == false?)

>          s->clock_valid = false;
>  
> +        data.clock = s->clock;
> +        data.flags = 0;
> +        ret = kvm_vm_ioctl(kvm_state, KVM_SET_CLOCK, &data);
> +        if (ret < 0) {
> +            fprintf(stderr, "KVM_SET_CLOCK failed: %s\n", strerror(ret));
> +            abort();
> +        }
> +
>          if (!cap_clock_ctrl) {
>              return;
>          }
> @@ -84,6 +62,26 @@ static void kvmclock_vm_state_change(void *opaque, int running,
>                  return;
>              }
>          }
> +    } else {
> +        struct kvm_clock_data data;
> +        int ret;
> +
> +        if (s->clock_valid) {
> +            return;
> +        }
> +        ret = kvm_vm_ioctl(kvm_state, KVM_GET_CLOCK, &data);
> +        if (ret < 0) {
> +            fprintf(stderr, "KVM_GET_CLOCK failed: %s\n", strerror(ret));
> +            abort();
> +        }
> +        s->clock = data.clock;
> +
> +        /*
> +         * If the VM is stopped, declare the clock state valid to
> +         * avoid re-reading it on next vmsave (which would return
> +         * a different value). Will be reset when the VM is continued.
> +         */
> +        s->clock_valid = !runstate_is_running();

Here we know that !runstate_is_running() is true (we're in the else
branch of "if (running)") so it can just be "s->clock_valid = true".
This matches the false assignment in the other branch.  This is just a
nit, but it makes the code clearer.

Paolo

>      }
>  }
>  
> @@ -100,8 +98,6 @@ static const VMStateDescription kvmclock_vmsd = {
>      .version_id = 1,
>      .minimum_version_id = 1,
>      .minimum_version_id_old = 1,
> -    .pre_save = kvmclock_pre_save,
> -    .post_load = kvmclock_post_load,
>      .fields = (VMStateField[]) {
>          VMSTATE_UINT64(clock, KVMClockState),
>          VMSTATE_END_OF_LIST()
> --
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