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

[Wei Wang]

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 <address@hidden>
> > > > Signed-off-by: Liang Li <address@hidden>
> > > > CC: Michael S. Tsirkin <address@hidden>
> > > > CC: Dr. David Alan Gilbert <address@hidden>
> > > > CC: Juan Quintela <address@hidden>
> > > 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.

The potential issue I could observe here is that 
"dev->free_page_report_status" is read and written by the optimization 
thread, and it is also modified by the migration thread and reset via 
virtio_balloon_free_page_stop.

How about adding a QEMU SpinLock, like this:

virtio_balloon_poll_free_page_hints()
{

    while (1) {
        qemu_spin_lock();
        /* If the status has been changed to STOP or EXIT, or the VM is 
stopped, just exit */
        if (dev->free_page_report_status >= FREE_PAGE_REPORT_S_STOP || 
!runstate_is_running()) {
            qemu_spin_unlock();
            break;
        }
        ....
        qemu_spin_unlock();
    }
}


Best,
Wei