Re: kvm mmu tracing quirks

[Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>]

On 04/14/2011 12:12 AM, Avi Kivity wrote:
> On 04/13/2011 05:09 PM, Jan Kiszka wrote:
>> Hi Xiao,
>>
>> while tracing a guest with all events enabled, I notices some issues
>> with kvm mmu instrumentations. A critical one:
>>
>> TRACE_EVENT(
>>     kvm_mmu_audit,
>>     TP_PROTO(struct kvm_vcpu *vcpu, int audit_point),
>>     TP_ARGS(vcpu, audit_point),
>>
>>     TP_STRUCT__entry(
>>         __field(struct kvm_vcpu *, vcpu)
>>         __field(int, audit_point)
>>     ),
>>
>>     TP_fast_assign(
>>         __entry->vcpu = vcpu;
>>         __entry->audit_point = audit_point;
>>     ),
>>
>>     TP_printk("vcpu:%d %s", __entry->vcpu->cpu,
>>           audit_point_name[__entry->audit_point])
>> );
>>
>> Saving the vcpu reference to the trace buffer can break on dump if the
>> vcpu was destroyed in the meantime. I was about to fix that by saving
>> vcpu->cpu instead, but then I wondered what kind of information you
>> actually need here. The triggering host cpu is automatically recorded by
>> ftrace anyway. Can you comment on this / clean it up? Also, it would be
>> nice to use __print_symbolic for translating audit_point to a string as
>> that would also work for trace-cmd/kernelshark.
>>

Thanks you for pointing it out, Jan! The fix is good for me!

> 
> IIRC the audit trace is only used to get a low cost invocation for the audit machinery; it's not actually useful as a user tracepoint.
> 

Yes, it is.

> We should probably switch to static_branch() instead (https://lwn.net/Articles/429447/).
> 

It's a good idea, i'll do that after static_branch merged!