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

[Peter Oskolkov]

On Thu, Apr 21, 2022 at 10:14 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> 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.

So this function:

uint8_t hyperchannel_ping(uint8_t arg)
{
        uint8_t inb;
        uint16_t port = PORT;

        asm(
                "outb %[arg] , %[port]  \n\t"  // write arg
                "inb  %[port], %[inb]   \n\t"  // read  res
                : [inb] "=r"(inb)
                : [arg] "r"(arg), [port] "r"(port)
        );
        return inb;
}

takes about 5.5usec vs 2.5usec for a vmcall on the same
hardware/kernel/etc. I've also tried AF_VSOCK, and a roundtrip there
is 30-50usec.

The main problem of port I/O vs a vmcall is that with port I/O a
second VM exit is needed to return any result to the guest. Am I
missing something?

I'll try now using ioeventfd, but I suspect that building a
synchronous request/response channel on top of it will not match a
direct vmcall in terms of latency.

Are there any other alternatives I should look at?

Thanks,
Peter

>
> 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:
>