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:
>
