Re: [PATCH 0/2] Expose KVM API to Linux Kernel

[Paolo Bonzini]

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