linux-kernel - Re: [PATCH] Add a page cache-backed balloon device driver.

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Date:	Mon, 03 Sep 2012 08:35:27 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Frank Swiderski <fes@...gle.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Rafael Aquini <aquini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mikew@...gle.com,
	Ying Han <yinghan@...gle.com>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] Add a page cache-backed balloon device driver.

Il 02/07/2012 02:29, Rusty Russell ha scritto:
> VIRTIO_BALLOON_F_MUST_TELL_HOST
> implies you should tell the host (eventually).  I don't know if any
> implementations actually care though.

This is indeed broken, because it is a "negative" feature: it tells you
that "implicit deflate" is _not_ supported.

Right now, QEMU refuses migration if the target does not support all the
features that were negotiated.  But then:

- a migration from non-MUST_TELL_HOST to MUST_TELL_HOST will succeed,
which is wrong;

- a migration from MUST_TELL_HOST to non-MUST_TELL_HOST will fail, which
is useless.

> We could add a VIRTIO_BALLOON_F_NEVER_TELL_DEFLATE which would mean the
> deflate vq need not be used at all.

That would work.  At the same time we could deprecate MUST_TELL_HOST.
Certainly the guest implementations don't care, or we would have
experienced problems such as the one above.  The QEMU implementation
also does not care but, for example, a Xen implementation would care.

Paolo
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