Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble hinting"

[Alexander Duyck <alexander.h.duyck@linux.intel.com>]

On Wed, 2019-07-24 at 15:02 -0400, Michael S. Tsirkin wrote:
> On Wed, Jul 24, 2019 at 10:12:10AM -0700, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > 
> > Add support for what I am referring to as "bubble hinting". Basically the
> > idea is to function very similar to how the balloon works in that we
> > basically end up madvising the page as not being used. However we don't
> > really need to bother with any deflate type logic since the page will be
> > faulted back into the guest when it is read or written to.
> > 
> > This is meant to be a simplification of the existing balloon interface
> > to use for providing hints to what memory needs to be freed. I am assuming
> > this is safe to do as the deflate logic does not actually appear to do very
> > much other than tracking what subpages have been released and which ones
> > haven't.
> > 
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > ---
> >  hw/virtio/virtio-balloon.c                      |   40 +++++++++++++++++++++++
> >  include/hw/virtio/virtio-balloon.h              |    2 +
> >  include/standard-headers/linux/virtio_balloon.h |    1 +
> >  3 files changed, 42 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> > index 2112874055fb..70c0004c0f88 100644
> > --- a/hw/virtio/virtio-balloon.c
> > +++ b/hw/virtio/virtio-balloon.c
> > @@ -328,6 +328,39 @@ static void balloon_stats_set_poll_interval(Object *obj, Visitor *v,
> >      balloon_stats_change_timer(s, 0);
> >  }
> >  
> > +static void virtio_bubble_handle_output(VirtIODevice *vdev, VirtQueue *vq)
> > +{
> > +    VirtQueueElement *elem;
> > +
> > +    while ((elem = virtqueue_pop(vq, sizeof(VirtQueueElement)))) {
> > +    	unsigned int i;
> > +
> > +        for (i = 0; i < elem->in_num; i++) {
> > +            void *addr = elem->in_sg[i].iov_base;
> > +            size_t size = elem->in_sg[i].iov_len;
> > +            ram_addr_t ram_offset;
> > +            size_t rb_page_size;
> > +            RAMBlock *rb;
> > +
> > +            if (qemu_balloon_is_inhibited())
> > +                continue;
> > +
> > +            rb = qemu_ram_block_from_host(addr, false, &ram_offset);
> > +            rb_page_size = qemu_ram_pagesize(rb);
> > +
> > +            /* For now we will simply ignore unaligned memory regions */
> > +            if ((ram_offset | size) & (rb_page_size - 1))
> > +                continue;
> > +
> > +            ram_block_discard_range(rb, ram_offset, size);
> 
> I suspect this needs to do like the migration type of
> hinting and get disabled if page poisoning is in effect.
> Right?

Shouldn't something like that end up getting handled via
qemu_balloon_is_inhibited, or did I miss something there? I assumed cases
like that would end up setting qemu_balloon_is_inhibited to true, if that
isn't the case then I could add some additional conditions. I would do it
in about the same spot as the qemu_balloon_is_inhibited check.