Linux Virtualization - Apr 2020 - [virtio-dev] Re: [PATCH] virtio-balloon: Disable free page hinting/reporting if page poison is disabled

On 17.04.20 12:19, Michael S. Tsirkin wrote:
> On Fri, Apr 17, 2020 at 12:09:38PM +0200, David Hildenbrand wrote:
>>  > What do you call "hinting ends" though? The fact we put
>>> a page in the VQ is not a guarantee that it's been consumed
>>> by the hypervisor.
>>>
>>
>> I'd say hinting ends once the hypervisor sets
FREE_PAGE_REPORT_S_DONE.
> 
> Can't find that one anywhere. what did I miss?
Sorry, the QEMU implementation is confusing. FREE_PAGE_REPORT_S_DONE is
translated to VIRTIO_BALLOON_CMD_ID_DONE

QEMU:

hw/virtio/virtio-balloon.c:virtio_balloon_free_page_report_notify()
-> virtio_balloon_free_page_done(dev)
-> s->free_page_report_status = FREE_PAGE_REPORT_S_DONE;
   virtio_notify_config(vdev);

When the guest reads the config
hw/virtio/virtio-balloon.c:virtio_balloon_get_config()
-> if (dev->free_page_report_status == FREE_PAGE_REPORT_S_DONE)
-> config.free_page_report_cmd_id = ... VIRTIO_BALLOON_CMD_ID_DONE


Linux:

drivers/virtio/virtio_balloon.c:report_free_page_func()
-> if (cmd_id_received == VIRTIO_BALLOON_CMD_ID_DONE) {
-> return_free_pages_to_mm()


So it's VIRTIO_BALLOON_CMD_ID_DONE.


-- 
Thanks,

David / dhildenb

On Fri, Apr 17, 2020 at 12:26:24PM +0200, David Hildenbrand
wrote:
> On 17.04.20 12:19, Michael S. Tsirkin wrote:
> > On Fri, Apr 17, 2020 at 12:09:38PM +0200, David Hildenbrand wrote:
> >>  > What do you call "hinting ends" though? The fact
we put
> >>> a page in the VQ is not a guarantee that it's been
consumed
> >>> by the hypervisor.
> >>>
> >>
> >> I'd say hinting ends once the hypervisor sets
FREE_PAGE_REPORT_S_DONE.
> > 
> > Can't find that one anywhere. what did I miss?
> 
> Sorry, the QEMU implementation is confusing. FREE_PAGE_REPORT_S_DONE is
> translated to VIRTIO_BALLOON_CMD_ID_DONE
Well VIRTIO_BALLOON_CMD_ID_DONE just means "don't give me any
more hints, I finished migration".
Guest will stop hinting even without that once it scans all
free memory.


> QEMU:
> 
> hw/virtio/virtio-balloon.c:virtio_balloon_free_page_report_notify()
> -> virtio_balloon_free_page_done(dev)
> -> s->free_page_report_status = FREE_PAGE_REPORT_S_DONE;
>    virtio_notify_config(vdev);
> 
> When the guest reads the config
> hw/virtio/virtio-balloon.c:virtio_balloon_get_config()
> -> if (dev->free_page_report_status == FREE_PAGE_REPORT_S_DONE)
> -> config.free_page_report_cmd_id = ... VIRTIO_BALLOON_CMD_ID_DONE
> 
> 
> Linux:
> 
> drivers/virtio/virtio_balloon.c:report_free_page_func()
> -> if (cmd_id_received == VIRTIO_BALLOON_CMD_ID_DONE) {
> -> return_free_pages_to_mm()
> 
> 
> So it's VIRTIO_BALLOON_CMD_ID_DONE.
> 
> 
> -- 
> Thanks,
> 
> David / dhildenb

On 17.04.20 12:29, Michael S. Tsirkin wrote:
> On Fri, Apr 17, 2020 at 12:26:24PM +0200, David Hildenbrand wrote:
>> On 17.04.20 12:19, Michael S. Tsirkin wrote:
>>> On Fri, Apr 17, 2020 at 12:09:38PM +0200, David Hildenbrand wrote:
>>>>  > What do you call "hinting ends" though? The
fact we put
>>>>> a page in the VQ is not a guarantee that it's been
consumed
>>>>> by the hypervisor.
>>>>>
>>>>
>>>> I'd say hinting ends once the hypervisor sets
FREE_PAGE_REPORT_S_DONE.
>>>
>>> Can't find that one anywhere. what did I miss?
>>
>> Sorry, the QEMU implementation is confusing. FREE_PAGE_REPORT_S_DONE is
>> translated to VIRTIO_BALLOON_CMD_ID_DONE
> 
> Well VIRTIO_BALLOON_CMD_ID_DONE just means "don't give me any
> more hints, I finished migration".
> Guest will stop hinting even without that once it scans all
> free memory.
Yeah, that's the end of the whole process where you can be sure the host
processed all requests definetly.

-- 
Thanks,

David / dhildenb