Re: [PATCH 1/1] powerpc/rtas: Implement reentrant rtas call

[Leonardo Bras <leonardo@linux.ibm.com>]

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Hello Nathan, thanks for the feedback!

On Fri, 2020-04-10 at 14:28 -0500, Nathan Lynch wrote:
> Leonardo Bras <leonardo@linux.ibm.com> writes:
> > Implement rtas_call_reentrant() for reentrant rtas-calls:
> > "ibm,int-on", "ibm,int-off",ibm,get-xive" and  "ibm,set-xive".
> > 
> > On LoPAPR Version 1.1 (March 24, 2016), from 7.3.10.1 to 7.3.10.4,
> > items 2 and 3 say:
> > 
> > 2 - For the PowerPC External Interrupt option: The * call must be
> > reentrant to the number of processors on the platform.
> > 3 - For the PowerPC External Interrupt option: The * argument call
> > buffer for each simultaneous call must be physically unique.
> > 
> > So, these rtas-calls can be called in a lockless way, if using
> > a different buffer for each call.
> > 

> From the language in the spec it's clear that these calls are intended
> to be reentrant with respect to themselves, but it's less clear to me
> that they are safe to call simultaneously with respect to each other or
> arbitrary other RTAS methods.

In my viewpoint, being reentrant to themselves, without being reentrant
to others would be very difficult to do, considering the way the
rtas_call is crafted to work.

I mean, I have no experience in rtas code, it's my viewpoint. In my
thoughts there is something like this:

common_path -> selects function by token -> reentrant function
					|-> non-reentrant function

If there is one function that is reentrant, it means the common_path
and function selection by token would need to be reentrant too.

> > This can be useful to avoid deadlocks in crashing, where rtas-calls are
> > needed, but some other thread crashed holding the rtas.lock.
> 
> Are these calls commonly used in the crash-handling path? Is this
> addressing a real issue you've seen?
> 

Yes, I noticed deadlocks during crashes, like this one:
#0 arch_spin_lock
#1  lock_rtas () 
#2  rtas_call (token=8204, nargs=1, nret=1, outputs=0x0)
#3  ics_rtas_mask_real_irq (hw_irq=4100) 
#4  machine_kexec_mask_interrupts
#5  default_machine_crash_shutdown
#6  machine_crash_shutdown 
#7  __crash_kexec
#8  crash_kexec
#9  oops_end

On ics_rtas_mask_real_irq() we have both ibm_int_off and ibm_set_xive,
so it makes sense to also add ibm_int_on and ibm_get_xive as reentrant
too.

Full discussion available on this thread:
http://patchwork.ozlabs.org/project/linuxppc-dev/patch/20200401000020.590447-1-leonardo@linux.ibm.com/

> 
> > +/*
> > + * Used for reentrant rtas calls.
> > + * According to LoPAR documentation, only "ibm,int-on", "ibm,int-off",
> > + * "ibm,get-xive" and "ibm,set-xive" are currently reentrant.
> > + * Reentrant calls need their own rtas_args buffer, so not using rtas.args.
> > + */
> 
> Please use kernel-doc format in new code.

Sure, v2 is going to be fixed.

> 
> 
> > +int rtas_call_reentrant(int token, int nargs, int nret, int *outputs, ...)
> > +{
> > +	va_list list;
> > +	struct rtas_args rtas_args;
> > +
> > +	if (!rtas.entry || token == RTAS_UNKNOWN_SERVICE)
> > +		return -1;
> > +
> > +	va_start(list, outputs);
> > +	va_rtas_call_unlocked(&rtas_args, token, nargs, nret, list);
> > +	va_end(list);
> 
> No, I don't think you can place the RTAS argument buffer on the stack:
> 
>   7.2.7, Software Implementation Note:
>   | The OS must be aware that the effective address range for RTAS is 4
>   | GB when instantiated in 32-bit mode and the OS should not pass RTAS
>   | addresses or blocks of data which might fall outside of this range.

Agree, moved to PACA.

I will send a v2 soon, it will be a 2-patch patchset.

Best regards,
Leonardo Bras

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