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

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

On 02/21/2019 06:18 PM, Dr. David Alan Gilbert wrote:
> * Wei Wang (wei.w.wang@intel.com) wrote:
>> 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
> OK, I was going to check with mst for (b) because I prefer it after the
> spec changes have been merged, but since mst is OK with it, then we can
> merge especially with (a) already merged.

OK, thanks!

Best,
Wei