[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().
>