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