Hi Jakub:
On 2019/11/27 上午4:35, Jakub Kicinski wrote:
On Tue, 26 Nov 2019 19:07:26 +0900, Prashant Bhole wrote:
Note: This RFC has been sent to netdev as well as qemu-devel lists
This series introduces XDP offloading from virtio_net. It is based on
the following work by Jason Wang:
https://netdevconf.info/0x13/session.html?xdp-offload-with-virtio-net
Current XDP performance in virtio-net is far from what we can achieve
on host. Several major factors cause the difference:
- Cost of virtualization
- Cost of virtio (populating virtqueue and context switching)
- Cost of vhost, it needs more optimization
- Cost of data copy
Because of above reasons there is a need of offloading XDP program to
host. This set is an attempt to implement XDP offload from the guest.
This turns the guest kernel into a uAPI proxy.
BPF uAPI calls related to the "offloaded" BPF objects are forwarded
to the hypervisor, they pop up in QEMU which makes the requested call
to the hypervisor kernel. Today it's the Linux kernel tomorrow it may
be someone's proprietary "SmartNIC" implementation.
Why can't those calls be forwarded at the higher layer? Why do they
have to go through the guest kernel?
I think doing forwarding at higher layer have the following issues:
- Need a dedicated library (probably libbpf) but application may choose
to do eBPF syscall directly
- Depends on guest agent to work
- Can't work for virtio-net hardware, since it still requires a hardware
interface for carrying offloading information
- Implement at the level of kernel may help for future extension like
BPF object pinning and eBPF helper etc.
Basically, this series is trying to have an implementation of
transporting eBPF through virtio, so it's not necessarily a guest to
host but driver and device. For device, it could be either a virtual one
(as done in qemu) or a real hardware.
If kernel performs no significant work (or "adds value", pardon the
expression), and problem can easily be solved otherwise we shouldn't
do the work of maintaining the mechanism.
My understanding is that it should not be much difference compared to
other offloading technology.
The approach of kernel generating actual machine code which is then
loaded into a sandbox on the hypervisor/SmartNIC is another story.
We've considered such way, but actual machine code is not as portable as
eBPF bytecode consider we may want:
- Support migration
- Further offload the program to smart NIC (e.g through macvtap
passthrough mode etc).
Thanks
I'd appreciate if others could chime in.
