Re: [Qemu-devel] [qemu patch 2/2] kvmclock: reduce kvmclock difference on migration

[Paolo Bonzini]

On 14/11/2016 15:50, Marcelo Tosatti wrote:
> Well, i didnt want to mix the meaning of the variables:
> 
> +    /* whether machine supports reliable KVM_GET_CLOCK */
> +    bool mach_use_reliable_get_clock;
> +
> +    /* whether source host supported reliable KVM_GET_CLOCK */
> +    bool src_use_reliable_get_clock;
> 
> See the comments on top (later if you look at the variable, 
> then have to think: well it has one name, but its disabled 
> by that other path as well, so its more than its 
> name,etc...).
> 
>> I'm thinking "mach_use_reliable_get_clock is just for migration,
> 
> Thats whether the machine supports it. New machines have it enabled,
> olders don't.

Yes.

>> src_use_reliable_get_clock is the state". 
> 
> Thats whether the migration source supported it.

But it's not used only for migration.  It's used on every vmstate change
(running->stop and stop->running, isn't it?  I think that, apart from
the migration case, it's better to use s->clock if kvmclock is stable,
even on older machine types.

>>>>> +static bool kvmclock_src_use_reliable_get_clock(void *opaque)
>>>>> +{
>>>>> +    KVMClockState *s = opaque;
>>>>> +
>>>>> +    /*
>>>>> +     * On machine types that support reliable KVM_GET_CLOCK,
>>>>> +     * if host kernel does provide reliable KVM_GET_CLOCK,
>>>>> +     * set src_use_reliable_get_clock=true so that destination
>>>>> +     * avoids reading kvmclock from memory.
>>>>> +     */
>>>>> +    if (s->mach_use_reliable_get_clock && kvm_has_adjust_clock_stable()) 
>>>>> {
>>>>> +        s->src_use_reliable_get_clock = true;
>>>>> +    }
>>>>> +
>>>>> +    return s->src_use_reliable_get_clock;
>>>>> +}
>>>>
>>>> Here you can just return s->mach_use_reliable_get_clock. 
>>>
>>> mach_use_reliable_get_clock can be true but host might not support it.
>>
>> Yes, but the "needed" function is only required to avoid breaking
>> pc-i440fx-2.7 and earlier. 
> 
> "needed" is required so that the migration between:
> 
> SRC             DEST                BEHAVIOUR
> ~support        supports            on migration read from guest,
>                                     on stop/cont use
>                                     kvm_get_clock/kvm_set_clock
> 
> Destination does not use KVM_GET_CLOCK value (which is
> broken and should not be used).

If needed returns false, the destination will see
src_use_reliable_get_clock = false anyway.

If needed returns true, the destination can still see
src_use_reliable_get_clock = false if that's the value.

needed  src_use_reliable_get_clock      effect
false   false                           use kvmclock_current_nsec
false   true                            use kvmclock_current_nsec
true    false                           use kvmclock_current_nsec
true    true                            use s->clock


So the idea is:

- set s->clock and s->reliable_get_clock on every KVM_GET_CLOCK

- on migration source, use subsections and
s->mach_use_reliable_get_clock to avoid breaking migration on pre-2.8
machine types

- on migration destination, use .pre_load so that s->reliable_get_clock
is initialized to false on older machine types

>> If you return true here, you can still
>> migrate a "false" value for src_use_reliable_get_clock.
> 
> But the source only uses a reliable KVM_GET_CLOCK if 
> both conditions are true.
> 
> And the subsection is only needed if the source
> uses a reliable KVM_GET_CLOCK.

Yes, but the point of the subsection is just to avoid breaking migration
format.

>> It is the same as "is using masterclock", which is actually a stricter
>> condition than the KVM_CHECK_EXTENSION return value.  The right check to
>> use is whether masterclock is in use, 
> 
> Actually its "has a reliable KVM_GET_CLOCK" (which returns 
> get_kernel_clock() + (rdtsc() - tsc_timestamp), 
> 
> "broken KVM_GET_CLOCK" =  get_kernel_clock()

Yes.  And these end up being the same.

Paolo