Messages in this thread | | | Date | Mon, 03 Sep 2012 08:35:27 +0200 | From | Paolo Bonzini <> | Subject | Re: [PATCH] Add a page cache-backed balloon device driver. |
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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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