Re: [Qemu-devel] [PATCH v2 3/5] hpet 'driftfix': add fields to HPETTimer and VMStateDescription

[Avi Kivity <avi@redhat.com>]

On 04/11/2011 12:06 PM, Ulrich Obergfell wrote:
> >>  vmstate_hpet_timer = {
> >>            VMSTATE_UINT64(fsb, HPETTimer),
> >>            VMSTATE_UINT64(period, HPETTimer),
> >>            VMSTATE_UINT8(wrap_flag, HPETTimer),
> >>  + VMSTATE_UINT64_V(saved_period, HPETTimer, 3),
> >>  + VMSTATE_UINT64_V(ticks_not_accounted, HPETTimer, 3),
> >>  + VMSTATE_UINT32_V(irqs_to_inject, HPETTimer, 3),
> >>  + VMSTATE_UINT32_V(irq_rate, HPETTimer, 3),
> >>  + VMSTATE_UINT32_V(divisor, HPETTimer, 3),
> >
> >  We ought to be able to use a subsection keyed off of whether any ticks
> >  are currently accumulated, no?
>
>
> Anthony,
>
> I'm not sure if I understand your question correctly. Are you suggesting
> to migrate the driftfix-related state conditionally / only if there are
> any ticks accumulated in 'ticks_not_accounted' and 'irqs_to_inject' ?
>
> The size of the driftfix-related state is 28 bytes per timer and we have
> 32 timers per HPETState, i.e. 896 additional bytes per HPETState. With a
> maximum number of 8 HPET blocks (HPETState), this amounts to 7168 bytes.
> Hence, unconditional migration of the driftfix-related state should not
> cause significant additional overhead.
>

It's not about overhead.

> Maybe I missed something. Could you please explain which benefit you see
> in using a subsection ?

In the common case of there being no drift, you can migrate from a qemu 
that supports driftfix to a qemu that doesn't.

-- 
error compiling committee.c: too many arguments to function