Re: [PATCH v1 2/3] virtio-balloon: fix free page hinting check on unrealize

[David Hildenbrand]

On 18.05.20 17:20, Alexander Duyck wrote:
> On Mon, May 18, 2020 at 1:37 AM David Hildenbrand <address@hidden> wrote:
>>
>> Checking against guest features is wrong. We allocated data structures
>> based on host features. We can rely on "free_page_bh" as an indicator
>> whether to un-do stuff instead.
>>
>> Fixes: c13c4153f76d ("virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT")
>> Cc: Wei Wang <address@hidden>
>> Cc: Michael S. Tsirkin <address@hidden>
>> Cc: Philippe Mathieu-Daudé <address@hidden>
>> Cc: Alexander Duyck <address@hidden>
>> Signed-off-by: David Hildenbrand <address@hidden>
>> ---
>>  hw/virtio/virtio-balloon.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
>> index dc3b1067ab..a4fcf2d777 100644
>> --- a/hw/virtio/virtio-balloon.c
>> +++ b/hw/virtio/virtio-balloon.c
>> @@ -818,7 +818,7 @@ static void virtio_balloon_device_unrealize(DeviceState 
>> *dev)
>>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>>      VirtIOBalloon *s = VIRTIO_BALLOON(dev);
>>
>> -    if (virtio_balloon_free_page_support(s)) {
>> +    if (s->free_page_bh) {
>>          qemu_bh_delete(s->free_page_bh);
>>          virtio_balloon_free_page_stop(s);
>>          precopy_remove_notifier(&s->free_page_report_notify);
> 
> Would it make sense to apply the same change to
> virtio_balloon_device_reset and virtio_balloon_set_status? At least in

Good question ...

> the case of virtio_balloon_set_status it seems like you could possibly
> exploit it somehow as clearing the feature in the guest will prevent
> the toggling of the block_iothread value.

The guest cannot change features at random points in time. However, what
happens if we are stopped and trigger a system_reset? The the vdev
features are stale ... I'll play with it, see if this is required, and
add a separate patch if necessary!

> 
> Reviewed-by: Alexander Duyck <address@hidden>
> 

Thanks!

-- 
Thanks,

David / dhildenb