Re: [PATCH 00/15] perf: KVM: Fix, optimize, and clean up callbacks

[Peter Zijlstra]

On Fri, Aug 27, 2021 at 02:52:25PM +0800, Like Xu wrote:
> + STATIC BRANCH/CALL friends.
> 
> On 27/8/2021 8:57 am, Sean Christopherson wrote:
> > This started out as a small series[1] to fix a KVM bug related to Intel PT
> > interrupt handling and snowballed horribly.
> > 
> > The main problem being addressed is that the perf_guest_cbs are shared by
> > all CPUs, can be nullified by KVM during module unload, and are not
> > protected against concurrent access from NMI context.
> 
> Shouldn't this be a generic issue of the static_call() usage ?
> 
> At the beginning, we set up the static entry assuming perf_guest_cbs != NULL:
> 
> 	if (perf_guest_cbs && perf_guest_cbs->handle_intel_pt_intr) {
> 		static_call_update(x86_guest_handle_intel_pt_intr,
> 				   perf_guest_cbs->handle_intel_pt_intr);
> 	}
> 
> and then we unset the perf_guest_cbs and do the static function call like this:
> 
> DECLARE_STATIC_CALL(x86_guest_handle_intel_pt_intr,
> *(perf_guest_cbs->handle_intel_pt_intr));
> static int handle_pmi_common(struct pt_regs *regs, u64 status)
> {
> 		...
> 		if (!static_call(x86_guest_handle_intel_pt_intr)())
> 			intel_pt_interrupt();
> 		...
> }

You just have to make sure all static_call() invocations that started
before unreg are finished before continuing with the unload.
synchronize_rcu() can help with that.

This is module unload 101. Nothing specific to static_call().