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

[Sean Christopherson]

On Fri, Jul 12, 2024, Joel Fernandes wrote:
> On Fri, Jul 12, 2024 at 09:44:16AM -0700, Sean Christopherson wrote:
> > On Fri, Jul 12, 2024, Steven Rostedt wrote:
> > > On Fri, 12 Jul 2024 10:09:03 -0400
> > > Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> 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.
> 
> I am a bit confused by your statement Sean, because if a higher prio HOST
> thread wakes up on the vCPU thread's phyiscal CPU, then a VM-Exit should
> happen. That has nothing to do with IRQ delivery.  What am I missing?

Why does that require hooking VM-Exit?