virtio balloon poison feature

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

Hi Alexander, all.
I am looking at this old commit and I am confused:


commit 4749f03e72f8ff6e66f54e38e41a560014e1a0e3
Author: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Date:   Tue Aug 25 07:45:10 2020 -0700

    content: Document balloon feature page poison
    
    Page poison provides a way for the guest to notify the host that it is
    initializing or poisoning freed pages with some specific poison value. As a
    result of this we can infer a couple traits about the guest:
    
    1. Free pages will contain a specific pattern within the guest.
    2. Modifying free pages from this value may cause an error in the guest.
    3. Pages will be immediately written to by the driver when deflated.
    
    There are currently no existing features that make use of this data. In the
    upcoming feature free page reporting we will need to make use of this to
    identify if we can evict pages from the guest without causing data
    corruption.
    
    Add documentation for the page poison feature describing the basic
    functionality and requirements.
    
    Fixes: https://github.com/oasis-tcs/virtio-spec/issues/84
    Reviewed-by: Cornelia Huck <cohuck@redhat.com>
    Reviewed-by: David Hildenbrand <david@redhat.com>
    Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
    Signed-off-by: Cornelia Huck <cohuck@redhat.com>



here is what I do not understand: why does driver tell device that
it will poison the pages after deflate? device does not access them
anymore so why does it care? should this have been *before inflate*,
actually?

how was this envisioned to be used?


Thanks,
-- 
MST