Re: [Qemu-devel] [PATCH 09/10] piix: do not raise irq while loading vmstate

[Michael S. Tsirkin]

On Wed, Sep 10, 2014 at 11:58:34AM +0200, Paolo Bonzini wrote:
> Il 10/09/2014 12:50, Michael S. Tsirkin ha scritto:
> > OK, got this, thanks for the explanation!
> > So the reason i8259 might be out of sync is
> > because it's not yet deserialized.
> 
> Yes, especially the IMR/IRR/ISR fields.
> 
> > I think it's a good idea to put (at least the
> > last part) in the commit log.
> 
> Like this:
> 
>     This patch disables raising an irq while loading the state of PCI bridge.
>     Because the i8259 has not been deserialized yet, raising an interrupt
>     could bring the system out-of-sync with the migration source.  For 
> example,
>     the migration source could have masked the interrupt in the i8259. On the
>     destination, the i8259 device model would not know that yet and would
>     trigger an interrupt in the CPU.
>     
>     This patch eliminates raising an irq and just restores the calculated
>     state fields in post_load function.  Interrupt state will be deserialized
>     separately through the IRR field of the i8259.

Yes, thanks!
Except imho it's a bit better to s/raising/setting/ in the last paragraph.

> > Also it's updating irq state, not just raising irq,
> > that might be problematic, right?
> 
> Well, the i8259 is in the reset state so ISR=IRR=0, aka all IRQ lines 
> are known to be low.

By luck, yes.

>  But yes, in general it's the update that is 
> problematic.