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

[Julia Suvorova]

On Thu, Jan 23, 2020 at 3:17 PM David Hildenbrand <address@hidden> wrote:
>
> On 23.01.20 15:08, Julia Suvorova wrote:
> > On Thu, Jan 16, 2020 at 1:36 PM David Hildenbrand <address@hidden> 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 <address@hidden>
> >>> ---
> >>>  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.