Re: [Qemu-devel] [PATCH v11 7/7] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

[Wei Wang <wei.w.wang@intel.com>]

On 02/20/2019 09:12 PM, Dr. David Alan Gilbert wrote:
> * Wang, Wei W (wei.w.wang@intel.com) wrote:
>> On Friday, December 14, 2018 7:17 PM, Dr. David Alan Gilbert wrote:
>>>> On 12/14/2018 05:56 PM, Dr. David Alan Gilbert wrote:
>>>>> * Wei Wang (wei.w.wang@intel.com) wrote:
>>>>>> On 12/13/2018 11:45 PM, Dr. David Alan Gilbert wrote:
>>>>>>> * Wei Wang (wei.w.wang@intel.com) wrote:
>>>>>>>> The new feature enables the virtio-balloon device to receive
>>>>>>>> hints of guest free pages from the free page vq.
>>>>>>>>
>>>>>>>> A notifier is registered to the migration precopy notifier
>>>>>>>> chain. The notifier calls free_page_start after the migration
>>>>>>>> thread syncs the dirty bitmap, so that the free page
>>>>>>>> optimization starts to clear bits of free pages from the
>>>>>>>> bitmap. It calls the free_page_stop before the migration
>>>>>>>> thread syncs the bitmap, which is the end of the current round
>>>>>>>> of ram save. The free_page_stop is also called to stop the
>>> optimization in the case when there is an error occurred in the process of
>>> ram saving.
>>>>>>>> Note: balloon will report pages which were free at the time of this
>>> call.
>>>>>>>> As the reporting happens asynchronously, dirty bit logging
>>>>>>>> must be enabled before this free_page_start call is made.
>>>>>>>> Guest reporting must be disabled before the migration dirty bitmap
>>> is synchronized.
>>>>>>>> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
>>>>>>>> CC: Michael S. Tsirkin <mst@redhat.com>
>>>>>>>> CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
>>>>>>>> CC: Juan Quintela <quintela@redhat.com>
>>>>>>>> CC: Peter Xu <peterx@redhat.com>
>>>>>>> I think I'm OK for this from the migration side, I'd appreciate
>>>>>>> someone checking the virtio and aio bits.
>>>>>>>
>>>>>>> I'm not too sure how it gets switched on and off - i.e. if we
>>>>>>> get a nice new qemu on a new kernel, what happens when I try and
>>>>>>> migrate to the same qemu on an older kernel without these hints?
>>>>>>>
>>>>>> This feature doesn't rely on the host kernel. Those hints are
>>>>>> reported from the guest kernel.
>>>>>> So migration across different hosts wouldn't affect the use of this
>>> feature.
>>>>>> Please correct me if I didn't get your point.
>>>>> Ah OK, yes;  now what about migrating from new->old qemu with a new
>>>>> guest but old machine type?
>>>>>
>>>> I think normally, the source QEMU and destination QEMU should have the
>>>> same QEMU booting parameter. If the destination QEMU doesn't support
>>>> "--device virtio-balloon,free-page-hint=true", which the source QEMU
>>>> has, the destination side QEMU will fail to boot, and migration will
>>>> not happen then.
>>> Ah that's OK; as long as free-page-hint is false by default that will work fine.
>>>
>>> Dave
>>>
>> Hi Dave,
>>
>> Could we have this feature in QEMU 4.0 (freeze on Mar 12)?
> I think so; can you remind me where we're up to:
>    a) It looks like you've already got the kernel changes merged -
> correct?

Yes, they were already merged half year ago.

>    b) What about the virtio spec changes - where are they upto?

The spec changes are in progress. v1 were posted out, a v2 is in 
preparation.

>    c) Where are the other reviews upto - I think most are reviewed - is
> it just 7/7 that is missing the review-by?
7/7 is about the virtio changes, and Michael has given the reviewed-by:
http://lists.nongnu.org/archive/html/qemu-devel/2018-12/msg03732.html


Best,
Wei