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

[Sean Christopherson]

On Thu, Apr 28, 2022, Peter Oskolkov wrote:
> 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?

The intent of the port I/O approach is that it's just a kick, the actual data
payload is delivered via a different memory channel. 

  0. guest/host establish a memory channel, e.g. guest annouces address to host at boot
  1. guest writes parameters to the memory channel
  2. guest does port I/O to let the host know there's work to be done
  3. KVM exits to the host
  4. host does the work, fills memory with the response
  5. host does KVM_RUN to re-enter the guest
  6. KVM runs the guest
  7. guest reads the response from memory

This is what Paolo meant by "memory page".

Using an ioeventfd avoids the overhead of #3 and #5.  Instead of exiting to
userspace, KVM signals the ioeventfd to wake the userspace I/O thread and immediately
resumes the guest.  The catch is that if you want a synchronous response, the guest
will have to wait for the host I/O thread to service the request, at which point the
benefits of avoiding the exit to userspace are largely lost.

Things like virtio-net (and presumably other virtio devices?) take advantage of
ioeventfd by using a ring buffer, e.g. put a Tx payload in the buffer, kick the
host and move on.