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Subject: Re: [Qemu-devel] [PATCH] *** Vhost-pci RFC v2 ***


On Sun, Jun 19, 2016 at 10:19 AM Wei Wang <wei.w.wang@intel.com> wrote:
This RFC proposes a design of vhost-pci, which is a new virtio device type.
The vhost-pci device is used for inter-VM communication.

Before I send a more complete review of the spec, I have a few overall questions:

- this patch is for the virtio spec? Why not patch the spec directly (https://tools.oasis-open.org/version-control/browse/wsvn/virtio/trunk/) I expect several rfc iterations, so perhaps it's easier as plain text file for now (as a qemu patch to doc/specs). btw, I would limit the audience at qemu-devel for now.
- I think the virtio spec should limit itself to the hw device description, and virtioq messages. Not the backend implementation (the ipc details, client/server etc).
- If it could be made not pci-specific, a better name for the device could be simply "driver": the driver of a virtio device. Or the "slave" in vhost-user terminology - consumer of virtq. I think you prefer to call it "backend" in general, but I find it more confusing.
- regarding the socket protocol, why not reuse vhost-user? it seems to me it supports most of what you need and more (like interrupt, migrations, protocol features, start/stop queues). Some of the extensions, like uuid, could be beneficial to vhost-user too.
- Why is it required or beneficial to support multiple "frontend" devices over the same "vhost-pci" device? It could simplify things if it was a single device. If necessary, that could also be interesting as a vhost-user extension.
- no interrupt support, I suppose you mainly looked at poll-based net devices
- when do you expect to share a wip/rfc implementation?


Changes in v2:
1. changed the vhost-pci driver to use a controlq to send acknowledgement
   messages to the vhost-pci server rather than writing to the device
   configuration space;

2. re-organized all the data structures and the description layout;

3. removed the VHOST_PCI_CONTROLQ_UPDATE_DONE socket message, which is redundant;

4. added a message sequence number to the msg info structure to identify socket
   messages, and the socket message exchange does not need to be blocking;

5. changed to used uuid to identify each VM rather than using the QEMU process

Wei Wang (1):
  Vhost-pci RFC v2: a new virtio device for inter-VM communication

 vhost-pci.patch | 341 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 341 insertions(+)
 create mode 100755 vhost-pci.patch


Marc-André Lureau

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