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Date: Fri, 15 Dec 2023 09:54:45 -0800
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Subject: Re: [RFC PATCH 0/8] Dynamic vcpu priority management in kvm
From: Sean Christopherson <seanjc@google.com>
To: Vineeth Remanan Pillai <vineeth@bitbyteword.org>
Cc: Ben Segall <bsegall@google.com>, Borislav Petkov <bp@alien8.de>, 
	Daniel Bristot de Oliveira <bristot@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>, 
	Dietmar Eggemann <dietmar.eggemann@arm.com>, "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>, 
	Juri Lelli <juri.lelli@redhat.com>, Mel Gorman <mgorman@suse.de>, 
	Paolo Bonzini <pbonzini@redhat.com>, Andy Lutomirski <luto@kernel.org>, 
	Peter Zijlstra <peterz@infradead.org>, Steven Rostedt <rostedt@goodmis.org>, 
	Thomas Gleixner <tglx@linutronix.de>, Valentin Schneider <vschneid@redhat.com>, 
	Vincent Guittot <vincent.guittot@linaro.org>, Vitaly Kuznetsov <vkuznets@redhat.com>, 
	Wanpeng Li <wanpengli@tencent.com>, Suleiman Souhlal <suleiman@google.com>, 
	Masami Hiramatsu <mhiramat@google.com>, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, 
	x86@kernel.org, Tejun Heo <tj@kernel.org>, Josh Don <joshdon@google.com>, 
	Barret Rhoden <brho@google.com>, David Vernet <dvernet@meta.com>, 
	Joel Fernandes <joel@joelfernandes.org>
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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).