[PATCH v4 0/2] Implement reentrant rtas call

[PATCH v4 0/2] Implement reentrant rtas call

[Leonardo Bras]

Patch 2 implement rtas_call_reentrant() for reentrant rtas-calls:
"ibm,int-on", "ibm,int-off",ibm,get-xive" and  "ibm,set-xive",
according to LoPAPR Version 1.1 (March 24, 2016).

For that, it's necessary that every call uses a different
rtas buffer (rtas_args). Paul Mackerras suggested using the PACA
structure for creating a per-cpu buffer for these calls.

Patch 1 was necessary to make PACA have a 'struct rtas_args' member.

Reentrant rtas calls can be 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>

---
Changes since v3:
- Adds protection from preemption and interruption

Changes since v2:
- Fixed build failure from ppc64e, by including spinlock_types.h on 
  rtas-types.h
- Improved commit messages

Changes since v1:
- Moved buffer from stack to PACA (as suggested by Paul Mackerras)
- Added missing output bits
- Improve documentation following kernel-doc format (as suggested by
  Nathan Lynch)


Leonardo Bras (2):
  powerpc/rtas: Move type/struct definitions from rtas.h into
    rtas-types.h
  powerpc/rtas: Implement reentrant rtas call

 arch/powerpc/include/asm/paca.h       |   2 +
 arch/powerpc/include/asm/rtas-types.h | 126 ++++++++++++++++++++++++++
 arch/powerpc/include/asm/rtas.h       | 119 +-----------------------
 arch/powerpc/kernel/rtas.c            |  42 +++++++++
 arch/powerpc/sysdev/xics/ics-rtas.c   |  22 ++---
 5 files changed, 183 insertions(+), 128 deletions(-)
 create mode 100644 arch/powerpc/include/asm/rtas-types.h

-- 
2.25.4

[PATCH v4 1/2] powerpc/rtas: Move type/struct definitions from rtas.h into rtas-types.h

[Leonardo Bras]

In order to get any rtas* struct into other headers, including rtas.h
may cause a lot of errors, regarding include dependency needed for
inline functions.

Create rtas-types.h and move there all type/struct definitions
from rtas.h, then include rtas-types.h into rtas.h.

Also, as suggested by checkpath.pl, replace uint8_t for u8, and keep
the same type pattern for the whole file, as they are the same
according to powerpc/boot/types.h.

Signed-off-by: Leonardo Bras <leobras.c@gmail.com>
---
 arch/powerpc/include/asm/rtas-types.h | 126 ++++++++++++++++++++++++++
 arch/powerpc/include/asm/rtas.h       | 118 +-----------------------
 2 files changed, 127 insertions(+), 117 deletions(-)
 create mode 100644 arch/powerpc/include/asm/rtas-types.h

diff --git a/arch/powerpc/include/asm/rtas-types.h b/arch/powerpc/include/asm/rtas-types.h
new file mode 100644
index 000000000000..87354e28f160
--- /dev/null
+++ b/arch/powerpc/include/asm/rtas-types.h
@@ -0,0 +1,126 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _POWERPC_RTAS_TYPES_H
+#define _POWERPC_RTAS_TYPES_H
+#ifdef __KERNEL__
+
+#include <linux/spinlock_types.h>
+
+typedef __be32 rtas_arg_t;
+
+struct rtas_args {
+	__be32 token;
+	__be32 nargs;
+	__be32 nret;
+	rtas_arg_t args[16];
+	rtas_arg_t *rets;     /* Pointer to return values in args[]. */
+};
+
+struct rtas_t {
+	unsigned long entry;		/* physical address pointer */
+	unsigned long base;		/* physical address pointer */
+	unsigned long size;
+	arch_spinlock_t lock;
+	struct rtas_args args;
+	struct device_node *dev;	/* virtual address pointer */
+};
+
+struct rtas_suspend_me_data {
+	atomic_t working; /* number of cpus accessing this struct */
+	atomic_t done;
+	int token; /* ibm,suspend-me */
+	atomic_t error;
+	struct completion *complete; /* wait on this until working == 0 */
+};
+
+struct rtas_error_log {
+	/* Byte 0 */
+	u8		byte0;			/* Architectural version */
+
+	/* Byte 1 */
+	u8		byte1;
+	/* XXXXXXXX
+	 * XXX		3: Severity level of error
+	 *    XX	2: Degree of recovery
+	 *      X	1: Extended log present?
+	 *       XX	2: Reserved
+	 */
+
+	/* Byte 2 */
+	u8		byte2;
+	/* XXXXXXXX
+	 * XXXX		4: Initiator of event
+	 *     XXXX	4: Target of failed operation
+	 */
+	u8		byte3;			/* General event or error*/
+	__be32		extended_log_length;	/* length in bytes */
+	unsigned char	buffer[1];		/* Start of extended log */
+						/* Variable length.      */
+};
+
+/* RTAS general extended event log, Version 6. The extended log starts
+ * from "buffer" field of struct rtas_error_log defined above.
+ */
+struct rtas_ext_event_log_v6 {
+	/* Byte 0 */
+	u8 byte0;
+	/* XXXXXXXX
+	 * X		1: Log valid
+	 *  X		1: Unrecoverable error
+	 *   X		1: Recoverable (correctable or successfully retried)
+	 *    X		1: Bypassed unrecoverable error (degraded operation)
+	 *     X	1: Predictive error
+	 *      X	1: "New" log (always 1 for data returned from RTAS)
+	 *       X	1: Big Endian
+	 *        X	1: Reserved
+	 */
+
+	/* Byte 1 */
+	u8 byte1;			/* reserved */
+
+	/* Byte 2 */
+	u8 byte2;
+	/* XXXXXXXX
+	 * X		1: Set to 1 (indicating log is in PowerPC format)
+	 *  XXX		3: Reserved
+	 *     XXXX	4: Log format used for bytes 12-2047
+	 */
+
+	/* Byte 3 */
+	u8 byte3;			/* reserved */
+	/* Byte 4-11 */
+	u8 reserved[8];			/* reserved */
+	/* Byte 12-15 */
+	__be32  company_id;		/* Company ID of the company	*/
+					/* that defines the format for	*/
+					/* the vendor specific log type	*/
+	/* Byte 16-end of log */
+	u8 vendor_log[1];		/* Start of vendor specific log	*/
+					/* Variable length.		*/
+};
+
+/* Vendor specific Platform Event Log Format, Version 6, section header */
+struct pseries_errorlog {
+	__be16 id;			/* 0x00 2-byte ASCII section ID	*/
+	__be16 length;			/* 0x02 Section length in bytes	*/
+	u8 version;			/* 0x04 Section version		*/
+	u8 subtype;			/* 0x05 Section subtype		*/
+	__be16 creator_component;	/* 0x06 Creator component ID	*/
+	u8 data[];			/* 0x08 Start of section data	*/
+};
+
+/* RTAS pseries hotplug errorlog section */
+struct pseries_hp_errorlog {
+	u8	resource;
+	u8	action;
+	u8	id_type;
+	u8	reserved;
+	union {
+		__be32	drc_index;
+		__be32	drc_count;
+		struct { __be32 count, index; } ic;
+		char	drc_name[1];
+	} _drc_u;
+};
+
+#endif /* __KERNEL__ */
+#endif /* _POWERPC_RTAS_TYPES_H */
diff --git a/arch/powerpc/include/asm/rtas.h b/arch/powerpc/include/asm/rtas.h
index 3c1887351c71..c35c5350b7e4 100644
--- a/arch/powerpc/include/asm/rtas.h
+++ b/arch/powerpc/include/asm/rtas.h
@@ -5,6 +5,7 @@
 
 #include <linux/spinlock.h>
 #include <asm/page.h>
+#include <asm/rtas-types.h>
 #include <linux/time.h>
 #include <linux/cpumask.h>
 
@@ -42,33 +43,6 @@
  *
  */
 
-typedef __be32 rtas_arg_t;
-
-struct rtas_args {
-	__be32 token;
-	__be32 nargs;
-	__be32 nret; 
-	rtas_arg_t args[16];
-	rtas_arg_t *rets;     /* Pointer to return values in args[]. */
-};  
-
-struct rtas_t {
-	unsigned long entry;		/* physical address pointer */
-	unsigned long base;		/* physical address pointer */
-	unsigned long size;
-	arch_spinlock_t lock;
-	struct rtas_args args;
-	struct device_node *dev;	/* virtual address pointer */
-};
-
-struct rtas_suspend_me_data {
-	atomic_t working; /* number of cpus accessing this struct */
-	atomic_t done;
-	int token; /* ibm,suspend-me */
-	atomic_t error;
-	struct completion *complete; /* wait on this until working == 0 */
-};
-
 /* RTAS event classes */
 #define RTAS_INTERNAL_ERROR		0x80000000 /* set bit 0 */
 #define RTAS_EPOW_WARNING		0x40000000 /* set bit 1 */
@@ -148,31 +122,6 @@ struct rtas_suspend_me_data {
 /* RTAS check-exception vector offset */
 #define RTAS_VECTOR_EXTERNAL_INTERRUPT	0x500
 
-struct rtas_error_log {
-	/* Byte 0 */
-	uint8_t		byte0;			/* Architectural version */
-
-	/* Byte 1 */
-	uint8_t		byte1;
-	/* XXXXXXXX
-	 * XXX		3: Severity level of error
-	 *    XX	2: Degree of recovery
-	 *      X	1: Extended log present?
-	 *       XX	2: Reserved
-	 */
-
-	/* Byte 2 */
-	uint8_t		byte2;
-	/* XXXXXXXX
-	 * XXXX		4: Initiator of event
-	 *     XXXX	4: Target of failed operation
-	 */
-	uint8_t		byte3;			/* General event or error*/
-	__be32		extended_log_length;	/* length in bytes */
-	unsigned char	buffer[1];		/* Start of extended log */
-						/* Variable length.      */
-};
-
 static inline uint8_t rtas_error_severity(const struct rtas_error_log *elog)
 {
 	return (elog->byte1 & 0xE0) >> 5;
@@ -212,47 +161,6 @@ uint32_t rtas_error_extended_log_length(const struct rtas_error_log *elog)
 
 #define RTAS_V6EXT_COMPANY_ID_IBM	(('I' << 24) | ('B' << 16) | ('M' << 8))
 
-/* RTAS general extended event log, Version 6. The extended log starts
- * from "buffer" field of struct rtas_error_log defined above.
- */
-struct rtas_ext_event_log_v6 {
-	/* Byte 0 */
-	uint8_t byte0;
-	/* XXXXXXXX
-	 * X		1: Log valid
-	 *  X		1: Unrecoverable error
-	 *   X		1: Recoverable (correctable or successfully retried)
-	 *    X		1: Bypassed unrecoverable error (degraded operation)
-	 *     X	1: Predictive error
-	 *      X	1: "New" log (always 1 for data returned from RTAS)
-	 *       X	1: Big Endian
-	 *        X	1: Reserved
-	 */
-
-	/* Byte 1 */
-	uint8_t byte1;			/* reserved */
-
-	/* Byte 2 */
-	uint8_t byte2;
-	/* XXXXXXXX
-	 * X		1: Set to 1 (indicating log is in PowerPC format)
-	 *  XXX		3: Reserved
-	 *     XXXX	4: Log format used for bytes 12-2047
-	 */
-
-	/* Byte 3 */
-	uint8_t byte3;			/* reserved */
-	/* Byte 4-11 */
-	uint8_t reserved[8];		/* reserved */
-	/* Byte 12-15 */
-	__be32  company_id;		/* Company ID of the company	*/
-					/* that defines the format for	*/
-					/* the vendor specific log type	*/
-	/* Byte 16-end of log */
-	uint8_t vendor_log[1];		/* Start of vendor specific log	*/
-					/* Variable length.		*/
-};
-
 static
 inline uint8_t rtas_ext_event_log_format(struct rtas_ext_event_log_v6 *ext_log)
 {
@@ -287,16 +195,6 @@ inline uint32_t rtas_ext_event_company_id(struct rtas_ext_event_log_v6 *ext_log)
 #define PSERIES_ELOG_SECT_ID_HOTPLUG		(('H' << 8) | 'P')
 #define PSERIES_ELOG_SECT_ID_MCE		(('M' << 8) | 'C')
 
-/* Vendor specific Platform Event Log Format, Version 6, section header */
-struct pseries_errorlog {
-	__be16 id;			/* 0x00 2-byte ASCII section ID	*/
-	__be16 length;			/* 0x02 Section length in bytes	*/
-	uint8_t version;		/* 0x04 Section version		*/
-	uint8_t subtype;		/* 0x05 Section subtype		*/
-	__be16 creator_component;	/* 0x06 Creator component ID	*/
-	uint8_t data[];			/* 0x08 Start of section data	*/
-};
-
 static
 inline uint16_t pseries_errorlog_id(struct pseries_errorlog *sect)
 {
@@ -309,20 +207,6 @@ inline uint16_t pseries_errorlog_length(struct pseries_errorlog *sect)
 	return be16_to_cpu(sect->length);
 }
 
-/* RTAS pseries hotplug errorlog section */
-struct pseries_hp_errorlog {
-	u8	resource;
-	u8	action;
-	u8	id_type;
-	u8	reserved;
-	union {
-		__be32	drc_index;
-		__be32	drc_count;
-		struct { __be32 count, index; } ic;
-		char	drc_name[1];
-	} _drc_u;
-};
-
 #define PSERIES_HP_ELOG_RESOURCE_CPU	1
 #define PSERIES_HP_ELOG_RESOURCE_MEM	2
 #define PSERIES_HP_ELOG_RESOURCE_SLOT	3
-- 
2.25.4

[PATCH v4 2/2] powerpc/rtas: Implement reentrant rtas call

[Leonardo Bras]

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.

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>
---
 arch/powerpc/include/asm/paca.h     |  2 ++
 arch/powerpc/include/asm/rtas.h     |  1 +
 arch/powerpc/kernel/rtas.c          | 52 +++++++++++++++++++++++++++++
 arch/powerpc/sysdev/xics/ics-rtas.c | 22 ++++++------
 4 files changed, 66 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/paca.h
index e3cc9eb9204d..5a76ba50b40f 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/rtas.c b/arch/powerpc/kernel/rtas.c
index c5fa251b8950..31710b358f44 100644
--- a/arch/powerpc/kernel/rtas.c
+++ b/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);
@@ -483,6 +484,57 @@ int rtas_call(int token, int nargs, int nret, int *outputs, ...)
 }
 EXPORT_SYMBOL(rtas_call);
 
+/**
+ * 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->reentrant_args;
+
+	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;
+}
+
 /* For RTAS_BUSY (-2), delay for 1 millisecond.  For an extended busy status
  * code of 990n, perform the hinted delay of 10^n (last digit) milliseconds.
  */
diff --git a/arch/powerpc/sysdev/xics/ics-rtas.c b/arch/powerpc/sysdev/xics/ics-rtas.c
index 6aabc74688a6..4cf18000f07c 100644
--- a/arch/powerpc/sysdev/xics/ics-rtas.c
+++ b/arch/powerpc/sysdev/xics/ics-rtas.c
@@ -50,8 +50,8 @@ static void ics_rtas_unmask_irq(struct irq_data *d)
 
 	server = xics_get_irq_server(d->irq, irq_data_get_affinity_mask(d), 0);
 
-	call_status = rtas_call(ibm_set_xive, 3, 1, NULL, hw_irq, server,
-				DEFAULT_PRIORITY);
+	call_status = rtas_call_reentrant(ibm_set_xive, 3, 1, NULL, hw_irq,
+					  server, DEFAULT_PRIORITY);
 	if (call_status != 0) {
 		printk(KERN_ERR
 			"%s: ibm_set_xive irq %u server %x returned %d\n",
@@ -60,7 +60,7 @@ static void ics_rtas_unmask_irq(struct irq_data *d)
 	}
 
 	/* Now unmask the interrupt (often a no-op) */
-	call_status = rtas_call(ibm_int_on, 1, 1, NULL, hw_irq);
+	call_status = rtas_call_reentrant(ibm_int_on, 1, 1, NULL, hw_irq);
 	if (call_status != 0) {
 		printk(KERN_ERR "%s: ibm_int_on irq=%u returned %d\n",
 			__func__, hw_irq, call_status);
@@ -91,7 +91,7 @@ static void ics_rtas_mask_real_irq(unsigned int hw_irq)
 	if (hw_irq == XICS_IPI)
 		return;
 
-	call_status = rtas_call(ibm_int_off, 1, 1, NULL, hw_irq);
+	call_status = rtas_call_reentrant(ibm_int_off, 1, 1, NULL, hw_irq);
 	if (call_status != 0) {
 		printk(KERN_ERR "%s: ibm_int_off irq=%u returned %d\n",
 			__func__, hw_irq, call_status);
@@ -99,8 +99,8 @@ static void ics_rtas_mask_real_irq(unsigned int hw_irq)
 	}
 
 	/* Have to set XIVE to 0xff to be able to remove a slot */
-	call_status = rtas_call(ibm_set_xive, 3, 1, NULL, hw_irq,
-				xics_default_server, 0xff);
+	call_status = rtas_call_reentrant(ibm_set_xive, 3, 1, NULL, hw_irq,
+					  xics_default_server, 0xff);
 	if (call_status != 0) {
 		printk(KERN_ERR "%s: ibm_set_xive(0xff) irq=%u returned %d\n",
 			__func__, hw_irq, call_status);
@@ -131,7 +131,7 @@ static int ics_rtas_set_affinity(struct irq_data *d,
 	if (hw_irq == XICS_IPI || hw_irq == XICS_IRQ_SPURIOUS)
 		return -1;
 
-	status = rtas_call(ibm_get_xive, 1, 3, xics_status, hw_irq);
+	status = rtas_call_reentrant(ibm_get_xive, 1, 3, xics_status, hw_irq);
 
 	if (status) {
 		printk(KERN_ERR "%s: ibm,get-xive irq=%u returns %d\n",
@@ -146,8 +146,8 @@ static int ics_rtas_set_affinity(struct irq_data *d,
 		return -1;
 	}
 
-	status = rtas_call(ibm_set_xive, 3, 1, NULL,
-			   hw_irq, irq_server, xics_status[1]);
+	status = rtas_call_reentrant(ibm_set_xive, 3, 1, NULL,
+				     hw_irq, irq_server, xics_status[1]);
 
 	if (status) {
 		printk(KERN_ERR "%s: ibm,set-xive irq=%u returns %d\n",
@@ -179,7 +179,7 @@ static int ics_rtas_map(struct ics *ics, unsigned int virq)
 		return -EINVAL;
 
 	/* Check if RTAS knows about this interrupt */
-	rc = rtas_call(ibm_get_xive, 1, 3, status, hw_irq);
+	rc = rtas_call_reentrant(ibm_get_xive, 1, 3, status, hw_irq);
 	if (rc)
 		return -ENXIO;
 
@@ -198,7 +198,7 @@ static long ics_rtas_get_server(struct ics *ics, unsigned long vec)
 {
 	int rc, status[2];
 
-	rc = rtas_call(ibm_get_xive, 1, 3, status, vec);
+	rc = rtas_call_reentrant(ibm_get_xive, 1, 3, status, vec);
 	if (rc)
 		return -1;
 	return status[0];
-- 
2.25.4

Re: [PATCH v4 2/2] powerpc/rtas: Implement reentrant rtas call

[Nicholas Piggin]

Excerpts from Leonardo Bras's message of May 15, 2020 9:51 am:
> 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.

What about rtas_call_unlocked? Do the callers need to take the rtas 
lock?

Machine checks must call ibm,nmi-interlock too, which we really don't 
want to take a lock for either. Hopefully that's in a class of its own
and we can essentially ignore with respect to other rtas calls.

The spec is pretty vague too :(

"The ibm,get-xive call must be reentrant to the number of processors on 
the platform."

This suggests ibm,get-xive can be called concurrently by multiple
processors. It doesn't say anything about being re-entrant against any 
of the other re-entrant calls. Maybe that could be reasonably assumed,
but I don't know if it's reasonable to assume it can be called 
concurrently with a *non-reentrant* call, is it?

> 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.

You can't do this, paca is not limited to RTAS_INSTANTIATE_MAX.
Which is good, because I didn't want you to add another 88 bytes to the 
paca :) Can you make it a pointer and allocate it separately? Check
the slb_shadow allocation, you could use a similar pattern.

The other option would be to have just one more rtas args, and have the 
crashing CPU always that. That would skirt the re-entrancy issue -- the
concurrency is only ever a last resort. Would be a bit tricker though.

Thanks,
Nick

Re: [PATCH v4 2/2] powerpc/rtas: Implement reentrant rtas call

[Nicholas Piggin]

Excerpts from Leonardo Bras's message of May 15, 2020 9:51 am:
> 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.

What about rtas_call_unlocked? Do the callers need to take the rtas 
lock?

Machine checks must call ibm,nmi-interlock too, which we really don't 
want to take a lock for either. Hopefully that's in a class of its own
and we can essentially ignore with respect to other rtas calls.

The spec is pretty vague too :(

"The ibm,get-xive call must be reentrant to the number of processors on 
the platform."

This suggests ibm,get-xive can be called concurrently by multiple
processors. It doesn't say anything about being re-entrant against any 
of the other re-entrant calls. Maybe that could be reasonably assumed,
but I don't know if it's reasonable to assume it can be called 
concurrently with a *non-reentrant* call, is it?

> 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.

You can't do this, paca is not limited to RTAS_INSTANTIATE_MAX.
Which is good, because I didn't want you to add another 88 bytes to the 
paca :) Can you make it a pointer and allocate it separately? Check
the slb_shadow allocation, you could use a similar pattern.

The other option would be to have just one more rtas args, and have the 
crashing CPU always that. That would skirt the re-entrancy issue -- the
concurrency is only ever a last resort. Would be a bit tricker though.

Thanks,
Nick

Re: [PATCH v4 2/2] powerpc/rtas: Implement reentrant rtas call

[Leonardo Bras]

Hello Nick,

On Fri, 2020-05-15 at 17:30 +1000, Nicholas Piggin wrote:
> Excerpts from Leonardo Bras's message of May 15, 2020 9:51 am:
> > 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.
> 
> What about rtas_call_unlocked? Do the callers need to take the rtas 
> lock?
> 
> Machine checks must call ibm,nmi-interlock too, which we really don't 
> want to take a lock for either. Hopefully that's in a class of its own
> and we can essentially ignore with respect to other rtas calls.
> 
> The spec is pretty vague too :(
> 
> "The ibm,get-xive call must be reentrant to the number of processors on 
> the platform."
> 
> This suggests ibm,get-xive can be called concurrently by multiple
> processors. It doesn't say anything about being re-entrant against any 
> of the other re-entrant calls. Maybe that could be reasonably assumed,
> but I don't know if it's reasonable to assume it can be called 
> concurrently with a *non-reentrant* call, is it?

This was discussed on a previous version of the patchset:

https://lore.kernel.org/linuxppc-dev/875zcy2v8o.fsf@linux.ibm.com/

He checked with partition firmware development and these calls can be
used concurrently with arbitrary other RTAS calls.

> 
> > 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.
> 
> You can't do this, paca is not limited to RTAS_INSTANTIATE_MAX.
> Which is good, because I didn't want you to add another 88 bytes to the 
> paca :) Can you make it a pointer and allocate it separately? Check
> the slb_shadow allocation, you could use a similar pattern.

Sure, I will send the next version with this change.

> 
> The other option would be to have just one more rtas args, and have the 
> crashing CPU always that. That would skirt the re-entrancy issue -- the
> concurrency is only ever a last resort. Would be a bit tricker though.

It seems a good idea, but I would like to try the previous alternative
first.

> Thanks,
> Nick

Thank you Nick! 

Re: [PATCH v4 2/2] powerpc/rtas: Implement reentrant rtas call

[Leonardo Bras]

Hello Nick,

On Fri, 2020-05-15 at 17:30 +1000, Nicholas Piggin wrote:
> Excerpts from Leonardo Bras's message of May 15, 2020 9:51 am:
> > 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.
> 
> What about rtas_call_unlocked? Do the callers need to take the rtas 
> lock?
> 
> Machine checks must call ibm,nmi-interlock too, which we really don't 
> want to take a lock for either. Hopefully that's in a class of its own
> and we can essentially ignore with respect to other rtas calls.
> 
> The spec is pretty vague too :(
> 
> "The ibm,get-xive call must be reentrant to the number of processors on 
> the platform."
> 
> This suggests ibm,get-xive can be called concurrently by multiple
> processors. It doesn't say anything about being re-entrant against any 
> of the other re-entrant calls. Maybe that could be reasonably assumed,
> but I don't know if it's reasonable to assume it can be called 
> concurrently with a *non-reentrant* call, is it?

This was discussed on a previous version of the patchset:

https://lore.kernel.org/linuxppc-dev/875zcy2v8o.fsf@linux.ibm.com/

He checked with partition firmware development and these calls can be
used concurrently with arbitrary other RTAS calls.

> 
> > 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.
> 
> You can't do this, paca is not limited to RTAS_INSTANTIATE_MAX.
> Which is good, because I didn't want you to add another 88 bytes to the 
> paca :) Can you make it a pointer and allocate it separately? Check
> the slb_shadow allocation, you could use a similar pattern.

Sure, I will send the next version with this change.

> 
> The other option would be to have just one more rtas args, and have the 
> crashing CPU always that. That would skirt the re-entrancy issue -- the
> concurrency is only ever a last resort. Would be a bit tricker though.

It seems a good idea, but I would like to try the previous alternative
first.

> Thanks,
> Nick

Thank you Nick!