Re: [PATCH v3] balloon: transitional device support

[Cornelia Huck <cornelia.huck@de.ibm.com>]

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?

> 
>  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},