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[Qemu-devel] Vhost-pci RFC2.0

From: Wang, Wei W
Subject: [Qemu-devel] Vhost-pci RFC2.0
Date: Wed, 19 Apr 2017 06:38:11 +0000


We made some design changes to the original vhost-pci design, and want to open
a discussion about the latest design (labelled 2.0) and its extension (2.1).
2.0 design: One VM shares the entire memory of another VM
2.1 design: One VM uses an intermediate memory shared with another VM for
                     packet transmission.

For the convenience of discussion, I have some pictures presented at this link:

Fig. 1 shows the common driver frame that we want use to build the 2.0 and 2.1
design. A TX/RX engine consists of a local ring and an exotic ring.
Local ring:
1) allocated by the driver itself;
2) registered with the device (i.e. virtio_add_queue())
Exotic ring:
1) ring memory comes from the outside (of the driver), and exposed to the driver
     via a BAR MMIO;
2) does not have a registration in the device, so no ioeventfd/irqfd, 
registers allocated in the device

Fig. 2 shows how the driver frame is used to build the 2.0 design.
1) Asymmetric: vhost-pci-net <-> virtio-net
2) VM1 shares the entire memory of VM2, and the exotic rings are the rings
    from VM2.
3) Performance (in terms of copies between VMs):
    TX: 0-copy (packets are put to VM2's RX ring directly)
    RX: 1-copy (the green arrow line in the VM1's RX engine)

Fig. 3 shows how the driver frame is used to build the 2.1 design.
1) Symmetric: vhost-pci-net <-> vhost-pci-net
2) Share an intermediate memory, allocated by VM1's vhost-pci device,
for data exchange, and the exotic rings are built on the shared memory
3) Performance:
    TX: 1-copy
RX: 1-copy

Fig. 4 shows the inter-VM notification path for 2.0 (2.1 is similar).
The four eventfds are allocated by virtio-net, and shared with vhost-pci-net:
Uses virtio-net's TX/RX kickfd as the vhost-pci-net's RX/TX callfd
Uses virtio-net's TX/RX callfd as the vhost-pci-net's RX/TX kickfd
Example of how it works:
After packets are put into vhost-pci-net's TX, the driver kicks TX, which
causes the an interrupt associated with fd3 to be injected to virtio-net

The draft code of the 2.0 design is ready, and can be found here:
Qemu: https://github.com/wei-w-wang/vhost-pci-device
Guest driver: https://github.com/wei-w-wang/vhost-pci-driver

We tested the 2.0 implementation using the Spirent packet
generator to transmit 64B packets, the results show that the
throughput of vhost-pci reaches around 1.8Mpps, which is around
two times larger than the legacy OVS+DPDK. Also, vhost-pci shows
better scalability than OVS+DPDK.


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