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

[Maxim Levitsky]

On Mon, 2020-05-18 at 13:18 +0200, Paolo Bonzini wrote:
> 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
> 
This can be used to run kernel drivers inside a very thin VM IMHO to break up the stigma,
that kernel driver is always a bad thing to and should be by all means replaced by a userspace driver,
something I see a lot lately, and what was the ground for rejection of my nvme-mdev proposal.


Best regards,
	Maxim Levitsky