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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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