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Subject: Re: [virtio-dev] Re: [PATCH v4 3/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT


On 03/15/2018 10:47 AM, Michael S. Tsirkin wrote:
On Thu, Mar 15, 2018 at 09:15:48AM +0800, Wei Wang wrote:
On 03/14/2018 10:12 PM, Michael S. Tsirkin wrote:
On Wed, Mar 14, 2018 at 02:03:19PM +0800, Wei Wang wrote:
On 03/14/2018 10:53 AM, Michael S. Tsirkin wrote:
On Wed, Mar 14, 2018 at 10:43:01AM +0800, Wei Wang wrote:
On 03/14/2018 12:49 AM, Michael S. Tsirkin wrote:
On Wed, Mar 07, 2018 at 08:34:24PM +0800, Wei Wang wrote:

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Signed-off-by: Liang Li <liang.z.li@intel.com>
CC: Michael S. Tsirkin <mst@redhat.com>
CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
CC: Juan Quintela <quintela@redhat.com>
I find it suspicious that neither unrealize nor reset
functions have been touched at all.
Are you sure you have thought through scenarious like
hot-unplug or disabling the device by guest?
OK. I think we can call balloon_free_page_stop in unrealize and reset.


+static void *virtio_balloon_poll_free_page_hints(void *opaque)
+{
+    VirtQueueElement *elem;
+    VirtIOBalloon *dev = opaque;
+    VirtQueue *vq = dev->free_page_vq;
+    uint32_t id;
+    size_t size;
What makes it safe to poke at this device from multiple threads?
I think that it would be safer to do it from e.g. BH.

Actually the free_page_optimization thread is the only user of free_page_vq,
and there is only one optimization thread each time. Would this be safe
enough?

Best,
Wei
Aren't there other fields there? Also things like reset affect all VQs.

Yes. But I think BHs are used to avoid re-entrancy, which isn't the issue
here.
Since you are adding locks to address the issue - doesn't this imply
reentrancy is exactly the issue?
Not really. The lock isn't intended for any reentrancy issues, since there
will be only one run of the virtio_balloon_poll_free_page_hints function at
any given time. Instead, the lock is used to synchronize
virtio_balloon_poll_free_page_hints and virtio_balloon_free_page_stop to
access dev->free_page_report_status.
I wonder whether that's enough. E.g. is there a race with guest
trying to reset the device? That resets all VQs you know.

I think that's OK - we will call virtio_balloon_free_page_stop in the device reset function, and qemu_thread_join() in virtio_balloon_free_page_stop will wait till the optimization thread exits. That is, the reset will proceed after the optimization thread exits.




Please see the whole picture below:

virtio_balloon_poll_free_page_hints()
{

     while (1) {
         qemu_spin_lock();
         if (dev->free_page_report_status >= FREE_PAGE_REPORT_S_STOP ||
             !runstate_is_running()) {
             qemu_spin_unlock();
             break;
         }
         ...
         if (id == dev->free_page_report_cmd_id) {
==>        dev->free_page_report_status = FREE_PAGE_REPORT_S_START;
         ...
         qemu_spin_unlock();
     }
}


static void virtio_balloon_free_page_stop(void *opaque)
{
     VirtIOBalloon *s = opaque;
     VirtIODevice *vdev = VIRTIO_DEVICE(s);

     qemu_spin_lock();
...
==>       s->free_page_report_status = FREE_PAGE_REPORT_S_STOP;
     ...
     qemu_spin_unlock();
}


Without the lock, there are theoretical possibilities that assigning STOP
below is overridden by START above. In that
case,virtio_balloon_free_page_stop does not effectively stop
virtio_balloon_poll_free_page_hints.
I think this issue couldn't be solved by BHs.

Best,
Wei
Don't all BHs run under the BQL?

Actually the virtio_balloon_free_page_stop is called by the migration thread (instead of a BH). Even we guarantee the migration thread calls virtio_balloon_free_page_stop under BQL, the BQL is still too big for our case. Imagine this case: when the migration thread calls virtio_balloon_free_page_stop to stop the reporting, it blocks by BQL as virtio_balloon_poll_free_page_hints is in progress with BQL held, and the migration thread won't proceed untill virtio_balloon_poll_free_page_hints exits (i.e. getting all the hints). I think this isn't our intention - we basically want the migration thread to stop the guest reporting immediately.
So I think the small lock above is better (it locks for only one hint).

Best,
Wei


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