Re: [PATCH v16.1 6/9] virtio-balloon: Add support for providing free page reports to host

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

On Tue, Feb 11, 2020 at 03:31:18PM +0100, David Hildenbrand wrote:
> On 11.02.20 15:07, Michael S. Tsirkin wrote:
> > On Tue, Feb 11, 2020 at 01:19:31PM +0100, David Hildenbrand wrote:
> >>>>
> >>>> Did you see the discussion regarding unifying handling of
> >>>> inflate/deflate/free_page_hinting_free_page_reporting, requested by
> >>>> Michael? I think free page reporting is special and shall be left alone.
> >>>
> >>> Not sure what do you mean by "left alone here". Could you clarify?
> >>
> >> Don't try to unify handling like I proposed below, because it's
> >> semantics are special.
> >>
> >>>
> >>>> VIRTIO_BALLOON_F_REPORTING is nothing but a more advanced inflate, right
> >>>> (sg, inflate based on size - not "virtio pages")?
> >>>
> >>>
> >>> Not exactly - it's also initiated by guest as opposed to host, and
> >>> not guided by the ballon size request set by the host.
> >>
> >> True, but AFAIKS you could use existing INFLATE/DEFLATE in a similar
> >> way. There is no way for the hypervisor to nack a request. The balloon
> >> size is not glued to inflate/deflate requests. The guests manually
> >> updates it.
> > 
> > Hmm how isn't it? num_pages is the only way to inflate/deflate.
> 
> Usually, guests are nice and respond to num_pages changes in an
> appropriate way, except:
> - Triggering deflate: Unload the driver. Suspend/hibernate. OOM.
>   (+ Reboot, although that's special)
> - Triggering inflate + deflate: Simple balloon compaction / page
>   migration.


These are all real situations but balloon always has been best effort.


> But that's not what I meant.
> 
> "actual" is updated by the guest, not by the host. So the "actual
> balloon size" is set by the guest. It's not glued to inflation/deflation
> requests. "num_pages" is the host request.

Well the expectation is that as long as guest has ample
available memory, when num_pages changes then
guest starts sending inflate/deflate requests,
until actual matches num_pages.

If it does not match, and we wait and it still doesn't,
then something unusual happened. People do depend on that
behaviour.

> AFAIKs, the guest could inflate/deflate (esp. temporarily) and
> communicate via "actual" the actual balloon size as he sees it.

OK so you want hinted but unused pages counted, and reported
in "actual"? That's a vmexit before each page use ...



> > Spec also says:
> > The device is driven either by the receipt of a configuration change notification, or by changing guest memory
> > needs, such as performing memory compaction or responding to out of memory conditions.
> > 
> > so ignoring compaction/oom (later is under-specified, not a good example
> > to follow) yes inflate/deflate are tied to host specified configuration
> Yes, "num_pages" is the host request. But I'd say the statement (esp.
> "the device is driven by") in the spec is rather weak. It does not
> explicitly state when inflation/deflation is allowed IMHO.

Right since it's all best effort anyway.


> -- 
> Thanks,
> 
> David / dhildenb