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

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Message-ID: <87sjawhr1r.fsf@rustcorp.com.au>
Date:	Thu, 06 Sep 2012 11:05:12 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Paolo Bonzini <pbonzini@...hat.com>
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.

Paolo Bonzini <pbonzini@...hat.com> writes:
> 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.

OK; I'm not sure we need to deprecate MUST_TELL_HOST, though since it's
never actually been used there's a good argument.

VIRTIO_BALLOON_F_SILENT_DEFLATE (or whatever it's called) would
obviously mean you couldn't ack VIRTIO_BALLOON_F_MUST_TELL_HOST.

Patches welcome!

Cheers,
Rusty.
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