Subject: Re: [PATCH 13/13] powerpc/rtas: place tracepoints in do_enter_rtas()

From: Michael Ellerman <[email-masked]>
Date: Tue, 29 Nov 2022 11:49:52 +1100

Nathan Lynch <[email-masked]> writes:
> "Nicholas Piggin" <[email-masked]> writes:
>> On Sat Nov 19, 2022 at 1:07 AM AEST, Nathan Lynch wrote:
>>> Call the just-added rtas tracepoints in do_enter_rtas(), taking care
>>> to avoid function name lookups in the CPU offline path.
>>>
>>> Signed-off-by: Nathan Lynch <[email-masked]>
>>> ---
>>>  arch/powerpc/kernel/rtas.c | 23 +++++++++++++++++++++++
>>>  1 file changed, 23 insertions(+)
>>>
>>> diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
>>> index 198366d641d0..3487b42cfbf7 100644
>>> --- a/arch/powerpc/kernel/rtas.c
>>> +++ b/arch/powerpc/kernel/rtas.c
>>> @@ -38,6 +38,7 @@
>>>  #include <asm/page.h>
>>>  #include <asm/rtas.h>
>>>  #include <asm/time.h>
>>> +#include <asm/trace.h>
>>>  #include <asm/udbg.h>
>>>  
>>>  enum rtas_function_flags {
>>> @@ -525,6 +526,7 @@ void enter_rtas(unsigned long);
>>>  static void do_enter_rtas(struct rtas_args *args)
>>>  {
>>>  	unsigned long msr;
>>> +	const char *name = NULL;
>>>  
>>>  	/*
>>>  	 * Make sure MSR[RI] is currently enabled as it will be forced later
>>> @@ -537,9 +539,30 @@ static void do_enter_rtas(struct rtas_args *args)
>>>  
>>>  	hard_irq_disable(); /* Ensure MSR[EE] is disabled on PPC64 */
>>>  
>>> +	if ((trace_rtas_input_enabled() || trace_rtas_output_enabled())) {
>>> +		/*
>>> +		 * rtas_token_to_function() uses xarray which uses RCU,
>>> +		 * but this code can run in the CPU offline path
>>> +		 * (e.g. stop-self), after it's become invalid to call
>>> +		 * RCU APIs.
>>> +		 */
>>
>> We can call this in real-mode via pseries_machine_check_realmode
>> -> fwnmi_release_errinfo, so tracing should be disabled for that
>> case too... Does this_cpu_set_ftrace_enabled(0) in the early
>> machine check handler cover that sufficiently?
>
> I suspect so, but I'd like to verify. Do you know how I could exercise
> this path in qemu or LPAR?

On a P9 or P10 LPAR you should be able to use tools/testing/selftests/powerpc/mce/inject-ra-err

cheers