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

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

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...

> 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).