Re: [RFC PATCH 0/8] Dynamic vcpu priority management in kvm

Sean Christopherson
2023-12-15T17:54:47Z

On Fri, Dec 15, 2023, Vineeth Remanan Pillai wrote:
> > You are basically proposing that KVM bounce-buffer data between guest and host.
> > I'm saying there's no _technical_ reason to use a bounce-buffer, just do zero copy.
> >
> I was also meaning zero copy only. The help required from the kvm side is:
> - Pass the address of the shared memory to bpf programs/scheduler once
> the guest sets it up.
> - Invoke scheduler registered callbacks on events like VMEXIT,
> VEMENTRY, interrupt injection etc. Its the job of guest and host
> paravirt scheduler to interpret the shared memory contents and take
> actions.
>
> I admit current RFC doesn't strictly implement hooks and callbacks -
> it calls sched_setscheduler in place of all callbacks that I mentioned
> above. I guess this was your strongest objection.

Ya, more or less.

> As you mentioned in the reply to Joel, if it is fine for kvm to allow
> hooks into events (VMEXIT, VMENTRY, interrupt injection etc) then, it
> makes it easier to develop the ABI I was mentioning and have the hooks
> implemented by a paravirt scheduler. We shall re-design the
> architecture based on this for v2.

Instead of going straight to a full blown re-design, can you instead post slightly
more incremental RFCs? E.g. flesh out enough code to get a BPF program attached
and receiving information, but do NOT wait until you have fully working setup
before posting the next RFC.

There are essentially four-ish things to sort out:

1. Where to insert/modify hooks in KVM
2. How KVM exposes KVM-internal information through said hooks
3. How a BPF program can influence the host scheduler
4. The guest/host ABI

#1 and #2 are largely KVM-only, and I think/hope we can get a rough idea of how
to address them before moving onto #3 and #4 (assuming #3 isn't already a solved
problem).