On Sun, Jul 03, 2011 at 12:46:59PM +0300, Sasha Levin wrote:
> On Sun, 2011-07-03 at 11:27 +0300, Michael S. Tsirkin wrote:
> > Doesn't this basically just revert
> > bf50e69f63d21091e525185c3ae761412be0ba72?
> >
>
> Yes. I haven't noticed that commit.
>
> Ignoring the VIRTIO_BALLOON_F_MUST_TELL_HOST feature flag causes an
> unnecessary delay due to having to kick and wait for the host to process
> the deflate vq before actually using the memory pages.
OTOH, as the commit points out:
Without this feature, we might free a page (and have another
user touch it) while the hypervisor is unprepared for it
Are you doing so many deflates that the speed is important?
> I've stumbled on it when writing the virtio_balloon driver for kvm
> tools. Initially my plan was to ignore the deflate vq completely, since
> the virtio spec says that unless we set the MUST_TELL_HOST flag,
> "deflation advice is merely a courtesy".
>
> I've noticed that if I don't process the deflate vq the guest doesn't
> use the deflated pages at all - something which contradicts the feature
> that lets him use the deflated pages without waiting for the deflate vq.
OK, so you have a host that does not process the vq.
As a result tell_host blocks forever.
With your patch, guest will block after releasing the
page, so the next deflate request will get stuck.
This doesn't seem to be a huge improvement so
we can conclude such a host is broken?
> >
> > On Sat, Jul 02, 2011 at 06:06:56AM +0300, Sasha Levin wrote:
> > > Unless the host requires that requested pages won't be used until
> > > he us notified (VIRTIO_BALLOON_F_MUST_TELL_HOST), only notify after
> > > deflating the balloon.
> > >
> > > This will avoid having to take an exit before actually using the pages.
> > >
> > > Cc: Rusty Russell <[email protected]>
> > > Cc: "Michael S. Tsirkin" <[email protected]>
> > > Cc: [email protected]
> > > Cc: [email protected]
> > > Signed-off-by: Sasha Levin <[email protected]>
> > > ---
> > > drivers/virtio/virtio_balloon.c | 16 ++++++++++------
> > > 1 files changed, 10 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/drivers/virtio/virtio_balloon.c
> > > b/drivers/virtio/virtio_balloon.c
> > > index e058ace..055f95d 100644
> > > --- a/drivers/virtio/virtio_balloon.c
> > > +++ b/drivers/virtio/virtio_balloon.c
> > > @@ -148,14 +148,18 @@ static void leak_balloon(struct virtio_balloon *vb,
> > > size_t num)
> > > vb->num_pages--;
> > > }
> > >
> > > -
> > > /*
> > > - * Note that if
> > > - * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
> > > - * is true, we *have* to do it in this order
> > > + * If the host doesn't require us to notify him before using
> > > + * pages which belong to the balloon, update him only after
> > > + * freeing those pages for guest use.
> > > */
> > > - tell_host(vb, vb->deflate_vq);
> > > - release_pages_by_pfn(vb->pfns, vb->num_pfns);
> > > + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST)) {
> > > + tell_host(vb, vb->deflate_vq);
> > > + release_pages_by_pfn(vb->pfns, vb->num_pfns);
> > > + } else {
> > > + release_pages_by_pfn(vb->pfns, vb->num_pfns);
> > > + tell_host(vb, vb->deflate_vq);
> > > + }
> > > }
> > >
> > > static inline void update_stat(struct virtio_balloon *vb, int idx,
> > > --
> > > 1.7.6
>
>
> --
>
> Sasha.
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