Re: [Qemu-devel] [virtio-dev] Vhost-pci RFC2.0

[Jason Wang <jasowang@redhat.com>]

On 2017年04月19日 14:38, Wang, Wei W wrote:
> Hi,
> 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:
> _https://github.com/wei-w-wang/vhost-pci-discussion/blob/master/vhost-pci-rfc2.0.pdf_

Hi, is there any doc or pointer that describes the the design in detail? 
E.g patch 4 in v1 
https://lists.gnu.org/archive/html/qemu-devel/2016-05/msg05163.html.

Thanks

> 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, 
> configuration
> 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.

Does this mean OVS+DPDK can only have ~0.9Mpps? A little bit surprise 
that the number looks rather low (I can get similar result if I use 
kernel bridge).

Thanks

> Also, vhost-pci shows
> better scalability than OVS+DPDK.
> Best,
> Wei