Re: [PATCH] KVM: x86: add HC_VMM_CUSTOM hypercall

[Paolo Bonzini]

On 4/21/22 18:51, Peter Oskolkov wrote:
> Allow kvm-based VMMs to request KVM to pass a custom vmcall
> from the guest to the VMM in the host.
>
> Quite often, operating systems research projects and/or specialized
> paravirtualized workloads would benefit from a extra-low-overhead,
> extra-low-latency guest-host communication channel.

You can use a memory page and an I/O port.  It should be as fast as a 
hypercall.  You can even change it to use ioeventfd if an asynchronous 
channel is enough, and then it's going to be less than 1 us latency.

Paolo

> With cloud-hypervisor modified to handle the new hypercall (simply
> return the sum of the received arguments), the following function in
> guest_userspace_  completes, on average, in 2.5 microseconds (walltime)
> on a relatively modern Intel Xeon processor: