Re: [PATCH v5 2/2] 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 <redacted>
---
 arch/powerpc/include/asm/paca.h     |  2 ++
 arch/powerpc/include/asm/rtas.h     |  1 +
 arch/powerpc/kernel/paca.c          | 20 +++++++++++
 arch/powerpc/kernel/rtas.c          | 52 +++++++++++++++++++++++++++++
 arch/powerpc/sysdev/xics/ics-rtas.c | 22 ++++++------
 5 files changed, 86 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/paca.h
index e3cc9eb9204d..87cd9c2220cc 100644
--- a/arch/powerpc/include/asm/paca.h
+++ b/arch/powerpc/include/asm/paca.h

@@ -29,6 +29,7 @@
 #include <asm/hmi.h>
 #include <asm/cpuidle.h>
 #include <asm/atomic.h>
+#include <asm/rtas-types.h>
 
 #include <asm-generic/mmiowb_types.h>
 

@@ -270,6 +271,7 @@ struct paca_struct {
 #ifdef CONFIG_MMIOWB
 	struct mmiowb_state mmiowb_state;
 #endif
+	struct rtas_args *reentrant_args;
 } ____cacheline_aligned;
 
 extern void copy_mm_to_paca(struct mm_struct *mm);

diff --git a/arch/powerpc/include/asm/rtas.h b/arch/powerpc/include/asm/rtas.h
index c35c5350b7e4..fa7509c85881 100644
--- a/arch/powerpc/include/asm/rtas.h
+++ b/arch/powerpc/include/asm/rtas.h

@@ -236,6 +236,7 @@ extern struct rtas_t rtas;
 extern int rtas_token(const char *service);
 extern int rtas_service_present(const char *service);
 extern int rtas_call(int token, int, int, int *, ...);
+int rtas_call_reentrant(int token, int nargs, int nret, int *outputs, ...);
 void rtas_call_unlocked(struct rtas_args *args, int token, int nargs,
 			int nret, ...);
 extern void __noreturn rtas_restart(char *cmd);

diff --git a/arch/powerpc/kernel/paca.c b/arch/powerpc/kernel/paca.c
index 3f91ccaa9c74..88c9b61489fc 100644
--- a/arch/powerpc/kernel/paca.c
+++ b/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 +165,23 @@ static struct slb_shadow * __init new_slb_shadow(int cpu, unsigned long limit)
 
 #endif /* CONFIG_PPC_BOOK3S_64 */
 
+/**
+ * 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.
+ *
+ * Return:	Pointer to allocated rtas_args
+ */
+static struct rtas_args * __init new_rtas_args(int cpu, unsigned long limit)
+{
+	limit = min_t(unsigned long, limit, RTAS_INSTANTIATE_MAX);
+
+	return alloc_paca_data(sizeof(struct rtas_args), L1_CACHE_BYTES,
+			       limit, cpu);
+}
+
 /* 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 +220,7 @@ 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
+	new_paca->reentrant_args = NULL;
 }
 
 /* Put the paca pointer into r13 and SPRG_PACA */

@@ -274,6 +293,7 @@ void __init allocate_paca(int cpu)
 #ifdef CONFIG_PPC_BOOK3S_64
 	paca->slb_shadow_ptr = new_slb_shadow(cpu, limit);
 #endif
+	paca->reentrant_args = new_rtas_args(cpu, limit);