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Subject: Re: [PATCH v3] balloon: transitional device support


On Mon, May 11, 2015 at 12:19:22PM +0200, Cornelia Huck wrote:
> On Tue, 28 Apr 2015 17:41:23 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > Virtio 1.0 cs02 doesn't include a modern balloon device.  At some
> > point we'll likely define an incompatible interface with a
> > different ID and different semantics.  But for now, it's not a
> > big effort to support a transitional balloon device: this has the
> > advantage of supporting existing drivers, transparently, as well
> > as transports that don't allow mixing virtio 0 and virtio 1
> > devices. And balloon is an easy device to test, so it's also
> > useful for people to test virtio core handling of transitional
> > devices.
> > 
> > Three issues with legacy hypervisors have been identified:
> > 1. Actual value is actually used, and is necessary for management
> >    to work. Luckily 4 byte config space writes are now atomic.
> >    When using old guests, hypervisors can detect access to the last byte.
> >    When using old hypervisors, drivers can use atomic 4-byte accesses.
> > 2. Hypervisors actually didn't ignore the stats from the first
> >    buffer supplied. This means the values there would be
> >    incorrect until hypervisor resends the request.
> >    Add a note suggesting hypervisors ignore the 1st buffer.
> > 3. QEMU simply over-writes stats from each buffer it gets.
> >    Thus if driver supplies a different subset of stats
> >    on each request, stale values will be there.
> >    Require drivers to supply the same subset on each
> >    request. This also gives us a simple way to figure out
> >    which stats are supported.
> > 
> > Additionally, this fixes a typo:
> > -  the driver SHOULD omit unsupported statistics.
> > should be:
> > +  devices omit unsupported statistics.
> > 
> > Note that we have a normative statement with this requirement
> > already, this is just general high-level description.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> > 
> > Changes from v2:
> > 	address comments by Cornelia and Rusty
> > 
> > 	since devices can use actual field for debugging,
> > 	make the requirement not to use it while driver
> > 	updates it a SHOULD NOT and not a MUST NOT.
> > 
> > 
> >  conformance.tex |  26 ++++++++-
> >  content.tex     | 173 +++++++++++++++++++++++++++++++++++++++++++++++---------
> >  2 files changed, 169 insertions(+), 30 deletions(-)
> > 
> 
> > diff --git a/content.tex b/content.tex
> > index 11015a5..036f38c 100644
> > --- a/content.tex
> > +++ b/content.tex
> > @@ -1,4 +1,4 @@
> > -\chapter{Basic Facilities of a Virtio Device}\label{sec:Basic Facilities of a Virtio Device}
> > +{Basic Facilities of a Virtio Device}\label{sec:Basic Facilities of a Virtio Device}
> 
> This hunk looks should not be in here?

Yes, good catch!
I'll drop it before I commit, but as the TC rules let us fix trivial
mistakes silently, I'm not going to repost just for this.

> > 
> >  A virtio device is discovered and identified by a bus-specific method
> >  (see the bus specific sections: \ref{sec:Virtio Transport Options / Virtio Over PCI Bus}~\nameref{sec:Virtio Transport Options / Virtio Over PCI Bus},


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