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

[Like Xu]

On 27/8/2021 3:44 pm, Peter Zijlstra wrote:
> 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.

Do you mean something like that:

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 64e310ff4f3a..e7d310af7509 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8465,6 +8465,7 @@ void kvm_arch_exit(void)
 #endif
 	kvm_lapic_exit();
 	perf_unregister_guest_info_callbacks(&kvm_guest_cbs);
+	synchronize_rcu();

 	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
 		cpufreq_unregister_notifier(&kvmclock_cpufreq_notifier_block,
diff --git a/kernel/events/core.c b/kernel/events/core.c
index e466fc8176e1..63ae56c5d133 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6508,6 +6508,7 @@ EXPORT_SYMBOL_GPL(perf_register_guest_info_callbacks);
 int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
 {
 	perf_guest_cbs = NULL;
+	arch_perf_update_guest_cbs();
 	return 0;
 }
 EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);

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