Re: [PATCH v2 2/4] virtio-mem: Skip most of virtio_mem_unplug_all() without plugged memory

[Juan Quintela <quintela@redhat.com>]

David Hildenbrand <david@redhat.com> wrote:
> On 06.07.23 10:15, Juan Quintela wrote:
>> David Hildenbrand <david@redhat.com> wrote:
>>> Already when starting QEMU we perform one system reset that ends up
>>> triggering virtio_mem_unplug_all() with no actual memory plugged yet.
>>> That, in turn will trigger ram_block_discard_range() and perform some
>>> other actions that are not required in that case.
>>>
>>> Let's optimize virtio_mem_unplug_all() for the case that no memory is
>>> plugged. This will be beneficial for x-ignore-shared support as well.
>>>
>>> Tested-by: Mario Casquero <mcasquer@redhat.com>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> It works, so ...
>> Reviewed-by: Juan Quintela <quintela@redhat.com>
>
> Thanks!
>
> [...]
>
>>> +        bitmap_clear(vmem->bitmap, 0, vmem->bitmap_size);
>>>           vmem->size = 0;
>>>           notifier_list_notify(&vmem->size_change_notifiers, &vmem->size);
>>>       }
>>> +
>>>       trace_virtio_mem_unplugged_all();
>>>       virtio_mem_resize_usable_region(vmem, vmem->requested_size, true);
>>>       return 0;
>> Once that we are here.  Do you remember _why_ do we allow virtio-mem
>> plug/unplug in the middle of a migration.
>> We forbid to plug/unplug everything else.  Why do we need to
>> plug/unplug
>> virtio-mem during migration?
>
> With virtio-mem you tell the VM the desired size for the device
> (requested-size), and the VM will select blocks to (un)plug and send
> (un)plug requests to the hypervisor in order to reach the requested
> size.
>
> So changing the requested size in the hypervisor (by the QEMU user)
> and the VM processing that resize request is asynchronous -- similar
> to memory ballooning.
>
> As the VM can send these (un)plug requests any time, and we exactly
> don't want to allow (un)plug during migration, we have
> virtio_mem_is_busy() to reject any such requests to tell the VM
> "please try again later".

Ahh.

I see it now.

Thanks, Juan.