Re: [PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: fes@google.com, aarcange@redhat.com, riel@redhat.com,
	yvugenfi@redhat.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, mikew@google.com,
	yinghan@google.com, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works
Date: Mon, 10 Sep 2012 09:03:59 +0300	[thread overview]
Message-ID: <20120910060359.GB16819@redhat.com> (raw)
In-Reply-To: <504D7F95.9070700@redhat.com>

On Mon, Sep 10, 2012 at 07:50:13AM +0200, Paolo Bonzini wrote:
> Il 09/09/2012 00:22, Michael S. Tsirkin ha scritto:
> >> 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?

> > Besides, there is no guarantee that virtqueue setup
> > happens after feature negotiation.
> 
> It is the only way that makes sense though (unless the guest would write
> 0 for its features).
>  Should we change that?

Not sure.  This was not always the case. Further
setup can fail with e.g ENOMEM and
driver could retry with a set of more conservative features.

I do think it would be nice to add a generic way for device to
notify guest about an internal failure.
This can only happen after DRIVER_OK status is written though,
and since existing drivers do not expect such failure, it might
be too late.

> >> 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?

...

> > 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.

-- 
MST