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

[Julia Suvorova <jusual@redhat.com>]

On Mon, Jan 27, 2020 at 11:54 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> 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.

You're right, it is a hack. I can add another handler to check if
device is powered on, and check it in have_balloon(),
In assumption that have_balloon means than we can use balloon when
it's presented, it is fine. But it will get more complicated once
hidden devices are here.

Best regards, Julia Suvorova.