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
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
