Re: [PATCH 05/15] perf: Track guest callbacks on a per-CPU basis

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Aug 27, 2021 at 02:49:50PM +0000, Sean Christopherson wrote:
> On Fri, Aug 27, 2021, Peter Zijlstra wrote:
> > On Thu, Aug 26, 2021 at 05:57:08PM -0700, Sean Christopherson wrote:
> > > Use a per-CPU pointer to track perf's guest callbacks so that KVM can set
> > > the callbacks more precisely and avoid a lurking NULL pointer dereference.
> > 
> > I'm completely failing to see how per-cpu helps anything here...
> 
> It doesn't help until KVM is converted to set the per-cpu pointer in flows that
> are protected against preemption, and more specifically when KVM only writes to
> the pointer from the owning CPU.  

So the 'problem' I have with this is that sane (!KVM using) people, will
still have to suffer that load, whereas with the static_call() we patch
in an 'xor %rax,%rax' and only have immediate code flow.

> Ignoring static call for the moment, I don't see how the unreg side can be safe
> using a bare single global pointer.  There is no way for KVM to prevent an NMI
> from running in parallel on a different CPU.  If there's a more elegant solution,
> especially something that can be backported, e.g. an rcu-protected pointer, I'm
> all for it.  I went down the per-cpu path because it allowed for cleanups in KVM,
> but similar cleanups can be done without per-cpu perf callbacks.

If all the perf_guest_cbs dereferences are with preemption disabled
(IRQs disabled, IRQ context, NMI context included), then the sequence:

	WRITE_ONCE(perf_guest_cbs, NULL);
	synchronize_rcu();

Ensures that all prior observers of perf_guest_csb will have completed
and future observes must observe the NULL value.