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

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Date: Fri, 15 Dec 2023 14:10:06 -0500
From: Vineeth Remanan Pillai <vineeth@...byteword.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Ben Segall <bsegall@...gle.com>, Borislav Petkov <bp@...en8.de>, 
	Daniel Bristot de Oliveira <bristot@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, 
	Juri Lelli <juri.lelli@...hat.com>, Mel Gorman <mgorman@...e.de>, 
	Paolo Bonzini <pbonzini@...hat.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Valentin Schneider <vschneid@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Vitaly Kuznetsov <vkuznets@...hat.com>, 
	Wanpeng Li <wanpengli@...cent.com>, Suleiman Souhlal <suleiman@...gle.com>, 
	Masami Hiramatsu <mhiramat@...gle.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	x86@...nel.org, Tejun Heo <tj@...nel.org>, Josh Don <joshdon@...gle.com>, 
	Barret Rhoden <brho@...gle.com>, David Vernet <dvernet@...a.com>, 
	Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [RFC PATCH 0/8] Dynamic vcpu priority management in kvm

On Fri, Dec 15, 2023 at 12:54 PM Sean Christopherson <seanjc@...gle.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