On Wed, Feb 05, 2020 at 06:52:34AM +0000, Wang, Wei W wrote:

On Wednesday, February 5, 2020 12:50 AM, Michael S. Tsirkin wrote:

quoted

quoted

Michael, any clue on which event we have to wait with
VIRTIO_BALLOON_F_MUST_TELL_HOST? IMHO, I don't think
VIRTIO_BALLOON_F_MUST_TELL_HOST applies to
VIRTIO_BALLOON_F_FREE_PAGE_HINT and we'd better document that. It

introduces complexity with no clear benefit.

I meant that we must wait for host to see the hint.

Why?

Best,
Wei

Well if we did the hint would be reliable, allowing host to immediately
drop any pages it gets in the hint. Originally I wanted to speed up
hinting by never waiting for host, but that does not seem to be what was
implemented: the only place we don't wait is the shrinker and it seems a
waste that we introduced complexity to host without getting any real
benefit out of it.

VIRTIO_BALLOON_F_MUST_TELL_HOST doesn't really apply to hinting
right now, so we could have used it to mean "hints must wait for host
to use buffers". I'm afraid it's already a wasted opportunity at
this point, reusing it isn't worth the compatibility headaches.

-- 
MST