Re: [virtio-comment] [PATCH v2 1/3] content: Document balloon feature free page hints

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 18 May 2020 13:15:50 -0700
Alexander Duyck <alexander.duyck@gmail.com> wrote:

> On Mon, May 18, 2020 at 6:18 AM David Hildenbrand <david@redhat.com> wrote:
> >
> > On 15.05.20 19:33, Alexander Duyck wrote:  

> > > +\subsubsection{Free Page Hinting}\label{sec:Device Types / Memory Balloon Device / Device Operation / Free Page Hinting}
> > > +
> > > +Free page hinting is designed to be used during migration to determine what
> > > +pages within the guest are currently unused so that they can be skipped over
> > > +while migrating the guest. The device will indicate that it is ready to start
> > > +performing hinting by setting the \field{free_page_hint_cmd_id} to one of the
> > > +non-reserved values that can be used as a command ID. The following values
> > > +are reserved:
> > > +
> > > +\begin{description}
> > > +\item[VIRTIO_BALLOON_CMD_ID_STOP (0)] Any command ID previously supplied by
> > > +  the device is invalid. The driver should halt all hinting until a new
> > > +  command ID is supplied.  
> >
> > Maybe "The driver should stop hinting free pages, but should not reuse
> > all previously hinted pages."  
> 
> The "reuse all previously hinted pages" seems rather unclear to me. I
> would like to make clear that in this case the "use" is the guest
> making use of the memory, not the driver doing something like
> recycling hints. So in the spots where you reference "the driver
> reusing pages" I think I might prefer to go with something along the
> lines of "releasing pages for use by the guest". The problem is that
> when you have a balloon we were referencing using pages from the
> balloon. Since we cannot reference the balloon I figure I will go with
> language where we "supply" and "release" hinted pages. That way we
> acknowledge that the driver is holding onto pages and not freeing them
> for use by the guest.
> 
> I'll probably go with something like:
>   The driver should stop hinting free pages, but
>   should not release any hinted pages for use by the guest.

"The driver should stop hinting free pages until a new command ID is
supplied, but should not release..."

would sound good to me.

> 
> > > +
> > > +\item[VIRTIO_BALLOON_CMD_ID_DONE (1)] Any command ID previously supplied by
> > > +  the device is invalid. The driver should halt all hinting and the hinting
> > > +  balloon can now be deflated returning all pages to the guest.  
> >
> > I would avoid the terminology "hinting balloon" and "deflation".  
> 
> So I will clean up the spots you pointed out and also work on the
> workflow down below that includes "hinting balloon" in the
> terminology.
> 
> > "The driver should stop hinting free pages and should reuse all
> > previously hinted pages.".  
> 
> Similar to the comment above:
>   The driver should stop hinting free pages, and
>   should release all hinted pages for use by the guest.

Also sounds good to me.


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