Re: [RFC PATCH v2 0/5] Paravirt Scheduling (Dynamic vcpu priority management)

[Sean Christopherson <seanjc@google.com>]

On Fri, Jul 12, 2024, Steven Rostedt wrote:
> On Fri, 12 Jul 2024 10:09:03 -0400
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
> > > 
> > > Steven Rostedt told me, what we instead need is a tracepoint callback in a
> > > driver, that does the boosting.  
> > 
> > I utterly dislike changing the system behavior through tracepoints. They were
> > designed to observe the system, not modify its behavior. If people start abusing
> > them, then subsystem maintainers will stop adding them. Please don't do that.
> > Add a notifier or think about integrating what you are planning to add into the
> > driver instead.
> 
> I tend to agree that a notifier would be much better than using
> tracepoints, but then I also think eBPF has already let that cat out of
> the bag. :-p
> 
> All we need is a notifier that gets called at every VMEXIT.

Why?  The only argument I've seen for needing to hook VM-Exit is so that the
host can speculatively boost the priority of the vCPU when deliverying an IRQ,
but (a) I'm unconvinced that is necessary, i.e. that the vCPU needs to be boosted
_before_ the guest IRQ handler is invoked and (b) it has almost no benefit on
modern hardware that supports posted interrupts and IPI virtualization, i.e. for
which there will be no VM-Exit.