Re: [PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works

On Mon, Sep 10, 2012 at 07:50:13AM +0200, Paolo Bonzini wrote:

Il 09/09/2012 00:22, Michael S. Tsirkin ha scritto:

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Almost.  One is "the guest, if really needed, can tell the host of
pages".  If not negotiated, and the host does not support it, the host
must break the guest (e.g. fail to offer any virtqueues).

There is no way in spec to break the guest.
You can not fail to offer virtqueues.

You can always return 0 for the first queue.

I don't think guest drivers recover gracefully from this.
Do they?

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Besides, there is no guarantee that virtqueue setup
happens after feature negotiation.

It is the only way that makes sense though (unless the guest would write
0 for its features).
 Should we change that?

Not sure.  This was not always the case. Further
setup can fail with e.g ENOMEM and
driver could retry with a set of more conservative features.

I do think it would be nice to add a generic way for device to
notify guest about an internal failure.
This can only happen after DRIVER_OK status is written though,
and since existing drivers do not expect such failure, it might
be too late.

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The other is "the guest, though, would prefer not to do so".  It is
different because the guest can proceed in a fallback mode even if the
host doesn't offer it.

I think I get what your proposed SILENT means what I do not get
is the motivation. It looks like a premature optimization to me.

The motivation is to let the driver choose between two behaviors: the
current one where ballooning is only done on request, and a more
aggressive one.

Yes but why is being silent any good? Optimization?
Any data to show that it will help some workload?

...

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OK so TELL says *when* to notify host, SILENT if set allows guest
to skip leak notifications? In this case TELL should just be ignored
when SILENT is set.

Yeah, that was my first idea.  However, there are existing drivers that
ignore SILENT, so that would not be 100% exact.

Not sure I follow the logic.
They don't ack SILENT so that would be 100% exact.

-- 
MST