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

[Sean Christopherson]

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.  

> > On x86, KVM supports being built as a module and thus can be unloaded.
> > And because the shared callbacks are referenced from IRQ/NMI context,
> > unloading KVM can run concurrently with perf, and thus all of perf's
> > checks for a NULL perf_guest_cbs are flawed as perf_guest_cbs could be
> > nullified between the check and dereference.
> 
> No longer allowing KVM to be a module would be *AWESOME*. I detest how
> much we have to export for KVM :/
> 
> Still, what stops KVM from doing a coherent unreg? Even the
> static_call() proposed in the other patch, unreg can do
> static_call_update() + synchronize_rcu() to ensure everybody sees the
> updated pointer (would require a quick audit to see all users are with
> preempt disabled, but I think your using per-cpu here already imposes
> the same).

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.

As for static calls, I certainly have no objection to employing static calls for
the callbacks, but IMO we should not be relying on static call for correctness,
i.e. the existing bug needs to be fixed first.