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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 29, 2019 at 09:58:04AM -0700, Alexander Duyck wrote:
> On Wed, Jul 24, 2019 at 1:42 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Wed, Jul 24, 2019 at 04:29:27PM -0400, Nitesh Narayan Lal wrote:
> > >
> > > On 7/24/19 4:18 PM, Alexander Duyck wrote:
> > > > 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.
> > > I don't think qemu_balloon_is_inhibited() will take care of the page poisoning
> > > situations.
> > > If I am not wrong we may have to look to extend VIRTIO_BALLOON_F_PAGE_POISON
> > > support as per Michael's suggestion.
> >
> >
> > BTW upstream qemu seems to ignore VIRTIO_BALLOON_F_PAGE_POISON ATM.
> > Which is probably a bug.
> > Wei, could you take a look pls?
> 
> So I was looking at sorting out this for the unused page reporting
> that I am working on and it occurred to me that I don't think we can
> do the free page hinting if any sort of poison validation is present.
> The problem is that free page hinting simply stops the page from being
> migrated. As a result if there was stale data present it will just
> leave it there instead of zeroing it or writing it to alternating 1s
> and 0s.

stale data where? on source or on destination?
do you mean the case where memory was corrupted?






> 
> Also it looks like the VIRTIO_BALLOON_F_PAGE_POISON feature is
> assuming that 0 means that page poisoning is disabled,
> when in reality
> it might just mean we are using the value zero to poison pages instead
> of the 0xaa pattern. As such I think there are several cases where we
> could incorrectly flag the pages with the hint and result in the
> migrated guest reporting pages that contain non-poison values.
> 


Well guest has this code:
static int virtballoon_validate(struct virtio_device *vdev)
{
        if (!page_poisoning_enabled())
                __virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_POISON);

        __virtio_clear_bit(vdev, VIRTIO_F_IOMMU_PLATFORM);
        return 0;
}

So it seems that host can figure out what is going on easily enough.
What did I miss?



> The zero assumption works for unused page reporting since we will be
> zeroing out the page when it is faulted back into the guest, however
> the same doesn't work for the free page hint since it is simply
> skipping the migration of the recently dirtied page.

Right but the dirtied page is normally full of 0 since that is the
poison value, if we just leave it there we still get 0s, right?

-- 
MST