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

From: Wei Wang
Subject: Re: [Qemu-devel] [PATCH v11 7/7] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT
Date: Fri, 22 Feb 2019 09:11:54 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 02/21/2019 06:18 PM, Dr. David Alan Gilbert wrote:
* Wei Wang (address@hidden) wrote:
On 02/20/2019 09:12 PM, Dr. David Alan Gilbert wrote:
* Wang, Wei W (address@hidden) 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 (address@hidden) wrote:
On 12/13/2018 11:45 PM, Dr. David Alan Gilbert wrote:
* Wei Wang (address@hidden) 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
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 <address@hidden>
CC: Michael S. Tsirkin <address@hidden>
CC: Dr. David Alan Gilbert <address@hidden>
CC: Juan Quintela <address@hidden>
CC: Peter Xu <address@hidden>
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
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.


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 -
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

    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:
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!


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