Re: [RFC v2 2/3] virtio: support delay of checks in virtio_load()

[Chuang Xu]

On 2022/12/13 上午4:18, Peter Xu wrote:

> On Tue, Dec 13, 2022 at 12:49:41AM +0800, Chuang Xu wrote:
> > +bool migration_enable_load_check_delay;
> >       
> I'm just afraid this is still too hacky.
>
> One thing is because this variable itself to be only set at specific phase
> during migration to cover that commit().  The other thing is I'm not sure
> we can always rely on the commit() being happen 100% - what if there's no
> memory layout changes throughout the whole process of vm load?  That'll be
> skipped if memory_region_update_pending==false as I said.
>     
Yes, you're right. I wanted to set memory_region_update_pending to true at 
the beginning of qemu_loadvm_state_main(), but somehow I forgot this detail..😭
> So far the best I can come up with is we allow each virtio device to
> register a vm state change handler (during virtio_load) to do the rest,
> then in the handler it unregisters itself so it only runs once right before
> the VM starts.  But I'm not sure whether the virtio developers will be
> happy with it.  Maybe worth a try.
>
> Feel free to have a look at like kvmvapic_vm_state_change() if you think
> that idea worth exploring.
>     
That's a good idea! 

But I don't think it's necessary to register a new vm state change handler. 
Maybe we just need to add a delay_check flag to VirtIODevice and do those 
delayed checks in virtio_vmstate_change() when delay_check is true.

Later I'll upload the v3 patches.

Thanks!