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

[Paolo Bonzini]

On 18/05/20 13:34, Maxim Levitsky wrote:
>> 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.
>
> 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.

It's a tought design decision between speeding up a kernel driver with
something like eBPF or wanting to move everything to userspace.

Networking has moved more towards the first because there are many more
opportunities for NIC-based acceleration, while storage has moved
towards the latter with things such as io_uring.  That said, I don't see
why in-kernel NVMeoF drivers would be acceptable for anything but Fibre
Channel (and that's only because FC HBAs try hard to hide most of the
SAN layers).

Paolo