From: Liu Jiang <gerry@xxxxxxxxxxxxxxxxx>
Userspace VMMs (e.g. Qemu microvm, Firecracker) take advantage of using
virtio over mmio devices as a lightweight machine model for modern
cloud. The standard virtio over MMIO transport layer only supports one
legacy interrupt, which is much heavier than virtio over PCI transport
layer using MSI. Legacy interrupt has long work path and causes specific
VMExits in following cases, which would considerably slow down the
performance:
1) read interrupt status register
2) update interrupt status register
3) write IOAPIC EOI register
We proposed to update virtio over MMIO to version 3[1] to add the
following new features and enhance the performance.
1) Support Message Signaled Interrupt(MSI), which increases the
interrupt performance for virtio multi-queue devices
2) Support per-queue doorbell, so the guest kernel may directly write
to the doorbells provided by virtio devices.
The following is the network tcp_rr performance testing report, tested
with virtio-pci device, vanilla virtio-mmio device and patched
virtio-mmio device (run test 3 times for each case):
netperf -t TCP_RR -H 192.168.1.36 -l 30 -- -r 32,1024
Virtio-PCI Virtio-MMIO Virtio-MMIO(MSI)
trans/s 9536 6939 9500
trans/s 9734 7029 9749
trans/s 9894 7095 9318
[1] https://lkml.org/lkml/2019/12/20/113