Re: [virtio-dev] Re: [PATCH] virtio-balloon: Disable free page hinting/reporting if page poison is disabled

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

On Fri, Apr 17, 2020 at 12:09:38PM +0200, David Hildenbrand wrote:
>  > What do you call "hinting ends" though? The fact we put
> > a page in the VQ is not a guarantee that it's been consumed
> > by the hypervisor.
> > 
> 
> I'd say hinting ends once the hypervisor sets FREE_PAGE_REPORT_S_DONE.

Can't find that one anywhere. what did I miss?



> > 
> > I think a strict definition is this:
> > - hint includes a command ID
> > - hint implies "page was unused at some point after guest reading command ID"
> > 
> > 
> > Hypervisor can use dirty tracking tricks to get from that to
> > "page is unused at the moment".
> > 
> >> Whereby X is
> >> currently assumed to be 0, correct?
> > 
> > 
> > 
> > Now we are talking about what's safe to do with the page.
> > 
> > If POISON flag is set by hypervisor but clear by guest,
> > or poison_val is 0, then it's clear it's safe to blow
> > away the page if we can figure out it's unused.
> > 
> > Otherwise, it's much less clear :)
> 
> Hah! Agreed :D
> 
> > 
> > 
> > 
> > I'll have to come back and re-read the rest next week, this
> > is complex stuff and I'm too rushed with other things today.
> 
> Yeah, I'm also loaded with other stuff. Maybe Alex has time to
> understand the details as well.
> 
> -- 
> Thanks,
> 
> David / dhildenb


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