Re: [PATCH 2/2] virtio-balloon: Reject qmp_balloon during hot-unplug

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

On Thu, Jan 23, 2020 at 04:46:18PM +0100, Julia Suvorova wrote:
> On Thu, Jan 23, 2020 at 3:17 PM David Hildenbrand <david@redhat.com> wrote:
> >
> > On 23.01.20 15:08, Julia Suvorova wrote:
> > > On Thu, Jan 16, 2020 at 1:36 PM David Hildenbrand <david@redhat.com> wrote:
> > >>
> > >> On 15.01.20 23:40, Julia Suvorova wrote:
> > >>> Hot-unplug takes some time due to communication with the guest.
> > >>> Do not change the device while freeing up resources.
> > >>>
> > >>> Signed-off-by: Julia Suvorova <jusual@redhat.com>
> > >>> ---
> > >>>  balloon.c                  | 2 +-
> > >>>  hw/virtio/virtio-balloon.c | 9 ++++++++-
> > >>>  include/sysemu/balloon.h   | 2 +-
> > >>>  3 files changed, 10 insertions(+), 3 deletions(-)
> > >>>
> > >>> diff --git a/balloon.c b/balloon.c
> > >>> index f104b42961..998ec53a0f 100644
> > >>> --- a/balloon.c
> > >>> +++ b/balloon.c
> > >>> @@ -119,5 +119,5 @@ void qmp_balloon(int64_t target, Error **errp)
> > >>>      }
> > >>>
> > >>>      trace_balloon_event(balloon_opaque, target);
> > >>> -    balloon_event_fn(balloon_opaque, target);
> > >>> +    balloon_event_fn(balloon_opaque, target, errp);
> > >>>  }
> > >>> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> > >>> index 57f3b9f22d..0fa4e4454b 100644
> > >>> --- a/hw/virtio/virtio-balloon.c
> > >>> +++ b/hw/virtio/virtio-balloon.c
> > >>> @@ -717,12 +717,19 @@ static void virtio_balloon_stat(void *opaque, BalloonInfo *info)
> > >>>                                               VIRTIO_BALLOON_PFN_SHIFT);
> > >>>  }
> > >>>
> > >>> -static void virtio_balloon_to_target(void *opaque, ram_addr_t target)
> > >>> +static void virtio_balloon_to_target(void *opaque, ram_addr_t target,
> > >>> +                                     Error **errp)
> > >>>  {
> > >>> +    DeviceState *bus_dev = qdev_get_bus_device(DEVICE(opaque));
> > >>>      VirtIOBalloon *dev = VIRTIO_BALLOON(opaque);
> > >>>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > >>>      ram_addr_t vm_ram_size = get_current_ram_size();
> > >>>
> > >>> +    if (bus_dev && bus_dev->pending_deleted_event) {
> > >>> +        error_setg(errp, "Hot-unplug of %s is in progress", vdev->name);
> > >>> +        return;
> > >>> +    }
> > >>> +
> > >>
> > >> How exactly does this help? The guest is free to inflate/deflate
> > >> whenever it wants.
> > >
> > > Guest is aware of hot-unplug start, and virtio driver should not
> > > initiate any operations. This patch just restricts issuing commands
> > > from qmp monitor.
> >
> > Why shouldn't the guest driver inflate/deflate while memory hotplug is
> > going on?
> >
> > Simple balloon compaction in a Linux guest -> deflate/inflate triggered
> > in the hypervisor.
> 
> QEMU crashes if inflate happens after powering-off PCI slot. Guest is
> unable to interact with virtio-balloon device then, driver is
> unloaded. But inflate can still happen if initiated from qmp.
> 
> Best regards, Julia Suvorova.

Hot-unplug in progress is probably a hack - we should
prevent access when device is powered off.
Also, it would appear that have_balloon is the correct place for this
kind of check.

-- 
MST