On Mon, Sep 10, 2012 at 08:38:09AM +0200, Paolo Bonzini wrote:

Il 10/09/2012 08:03, Michael S. Tsirkin ha scritto:

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

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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?

No, that's the point ("break the guest" is really "break the driver").

You can just stop VM then. No need for a side channel.

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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?

Idle guests can move cache pages to the balloon.  You can overcommit
more aggressively, because the host can madvise away a lot more memory.

IMHO this feature needs more thought. E.g. how will this work with assignment?
If we build something let's build it in a way that plays nicely
with other features.

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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.

Hmm, then I'm not sure I follow yours.  We agreed that delaying
notifications or skipping them is really the same thing, right?

I think we're just stuck in a linguistic problem, with "must not" being
wrong and "does not have to" being too verbose.  Calling it
VIRTIO_BALLOON_F_SILENT_DEFLATE was a workaround for this, but perhaps
it adds more confusion.

Paolo

Looks like it does :)