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

[Paolo Bonzini]

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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