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Subject: Version Control Commit by mstsirkin
Author: mstsirkin Date: 2015-05-18 20:48:58 +0000 (Mon, 18 May 2015) New Revision: 526 Web View: https://tools.oasis-open.org/version-control/browse/wsvn/virtio/?rev=526&sc=1 Modified: branches/v1.0/conformance.tex branches/v1.0/content.tex Log: balloon: transitional device support 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. VIRTIO-143 Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
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