powerpc/rtas: Implement reentrant rtas call

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 cpu doing such rtas call.

For this, it was suggested to add the buffer (struct rtas_args)
in the PACA struct, so each cpu can have it's own buffer.
The PACA struct received a pointer to rtas buffer, which is
allocated in the memory range available to rtas 32-bit.

Reentrant rtas calls are useful to avoid deadlocks in crashing,
where rtas-calls are needed, but some other thread crashed holding
the rtas.lock.

This is a backtrace of a deadlock from a kdump testing environment:

  #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

Signed-off-by: Leonardo Bras <leobras.c@gmail.com>
[mpe: Move under #ifdef PSERIES to avoid build breakage]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200518234245.200672-3-leobras.c@gmail.com

arch/powerpc/kernel/paca.c

@@ -16,6 +16,7 @@
 #include <asm/kexec.h>
 #include <asm/svm.h>
 #include <asm/ultravisor.h>
+#include <asm/rtas.h>
 
 #include "setup.h"
 
@@ -164,6 +165,30 @@ static struct slb_shadow * __init new_slb_shadow(int cpu, unsigned long limit)
 
 #endif /* CONFIG_PPC_BOOK3S_64 */
 
+#ifdef CONFIG_PPC_PSERIES
+/**
+ * new_rtas_args() - Allocates rtas args
+ * @cpu:	CPU number
+ * @limit:	Memory limit for this allocation
+ *
+ * Allocates a struct rtas_args and return it's pointer,
+ * if not in Hypervisor mode
+ *
+ * Return:	Pointer to allocated rtas_args
+ *		NULL if CPU in Hypervisor Mode
+ */
+static struct rtas_args * __init new_rtas_args(int cpu, unsigned long limit)
+{
+	limit = min_t(unsigned long, limit, RTAS_INSTANTIATE_MAX);
+
+	if (early_cpu_has_feature(CPU_FTR_HVMODE))
+		return NULL;
+
+	return alloc_paca_data(sizeof(struct rtas_args), L1_CACHE_BYTES,
+			       limit, cpu);
+}
+#endif /* CONFIG_PPC_PSERIES */
+
 /* The Paca is an array with one entry per processor.  Each contains an
  * lppaca, which contains the information shared between the
  * hypervisor and Linux.
@@ -202,6 +227,10 @@ void __init __nostackprotector initialise_paca(struct paca_struct *new_paca, int
 	/* For now -- if we have threads this will be adjusted later */
 	new_paca->tcd_ptr = &new_paca->tcd;
 #endif
+
+#ifdef CONFIG_PPC_PSERIES
+	new_paca->rtas_args_reentrant = NULL;
+#endif
 }
 
 /* Put the paca pointer into r13 and SPRG_PACA */
@@ -273,6 +302,9 @@ void __init allocate_paca(int cpu)
 #endif
 #ifdef CONFIG_PPC_BOOK3S_64
 	paca->slb_shadow_ptr = new_slb_shadow(cpu, limit);
+#endif
+#ifdef CONFIG_PPC_PSERIES
+	paca->rtas_args_reentrant = new_rtas_args(cpu, limit);
 #endif
 	paca_struct_size += sizeof(struct paca_struct);
 }

arch/powerpc/kernel/rtas.c

@@ -41,6 +41,7 @@
 #include <asm/time.h>
 #include <asm/mmu.h>
 #include <asm/topology.h>
+#include <asm/paca.h>
 
 /* This is here deliberately so it's only used in this file */
 void enter_rtas(unsigned long);
@@ -1014,6 +1015,57 @@ out:
 	free_cpumask_var(offline_mask);
 	return atomic_read(&data.error);
 }
+
+/**
+ * rtas_call_reentrant() - Used for reentrant rtas calls
+ * @token:	Token for desired reentrant RTAS call
+ * @nargs:	Number of Input Parameters
+ * @nret:	Number of Output Parameters
+ * @outputs:	Array of outputs
+ * @...:	Inputs for desired RTAS call
+ *
+ * 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, but
+ * PACA one instead.
+ *
+ * Return:	-1 on error,
+ *		First output value of RTAS call if (nret > 0),
+ *		0 otherwise,
+ */
+int rtas_call_reentrant(int token, int nargs, int nret, int *outputs, ...)
+{
+	va_list list;
+	struct rtas_args *args;
+	unsigned long flags;
+	int i, ret = 0;
+
+	if (!rtas.entry || token == RTAS_UNKNOWN_SERVICE)
+		return -1;
+
+	local_irq_save(flags);
+	preempt_disable();
+
+	/* We use the per-cpu (PACA) rtas args buffer */
+	args = local_paca->rtas_args_reentrant;
+
+	va_start(list, outputs);
+	va_rtas_call_unlocked(args, token, nargs, nret, list);
+	va_end(list);
+
+	if (nret > 1 && outputs)
+		for (i = 0; i < nret - 1; ++i)
+			outputs[i] = be32_to_cpu(args->rets[i + 1]);
+
+	if (nret > 0)
+		ret = be32_to_cpu(args->rets[0]);
+
+	local_irq_restore(flags);
+	preempt_enable();
+
+	return ret;
+}
+
 #else /* CONFIG_PPC_PSERIES */
 int rtas_ibm_suspend_me(u64 handle)
 {