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

From: "Nicholas Piggin" <[email-masked]>
Date: Tue, 29 Nov 2022 16:47:17 +1000

On Tue Nov 29, 2022 at 9:44 AM AEST, Nathan Lynch wrote:
> "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?

I have some machine check injection patches for qemu but they never got
merged... I can point you to them if you need.

Should try get those merged again.

Thanks,
Nick