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

Vineeth Remanan Pillai
2023-12-15T19:10:18Z

On Fri, Dec 15, 2023 at 12:54 PM Sean Christopherson <seanjc@google.com> wrote:
>
> 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.
>
Sure, makes sense.

> 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).

Agreed. Will start with the kvm side and keep you posted on the progress.

Thanks,
Vineeth