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

[David Hildenbrand <david@redhat.com>]

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".

-- 
Cheers,

David / dhildenb