On 18/05/20 10:45, Anastassios Nanos wrote: > Being in the kernel saves us from doing unneccessary mode switches. > Of course there are optimizations for handling I/O on QEMU/KVM VMs > (virtio/vhost), but essentially what happens is removing mode-switches (and > exits) for I/O operations -- is there a good reason not to address that > directly? a guest running in the kernel exits because of an I/O request, > which gets processed and forwarded directly to the relevant subsystem *in* > the kernel (net/block etc.). In high-performance configurations, most of the time virtio devices are processed in another thread that polls on the virtio rings. In this setup, the rings are configured to not cause a vmexit at all; this has much smaller latency than even a lightweight (kernel-only) vmexit, basically corresponding to writing an L1 cache line back to L2. Paolo
