[PATCH v3 2/2] s390/ap: Cap AP maximum message size adjustable via module parameter

[Harald Freudenberger <freude@linux.ibm.com>]

With AP bus now supporting the new APMLM field with PQAP/TAPQ the AP
max message size could be raised up to 960KB. As this amount of memory
is tried to get allocated on each AP request/reply it would quickly
cause some memory problems.

Introduce a reasonable default upper limit
AP_DEFAULT_MAX_MSG_SIZE_LIMIT (72KB) and expose it via a read-only
module parameter 'max_msg_size_limit'. When creating an AP card, clip
the calculated AP max message size from the hardware info fetched via
PQAP/TAPQ to this upper limit and log a debug feature warning
(AP_DBF_WARN) if the card's reported limit exceeds it.

Also define some lower limit AP_MIN_MSG_SIZE (12KB) and ensure during
module init that a user-supplied max_msg_size_limit parameter is not
lower than this minimum, adjusting it with a warning if necessary.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
---
 drivers/s390/crypto/ap_bus.c  | 11 +++++++++++
 drivers/s390/crypto/ap_bus.h  |  5 ++++-
 drivers/s390/crypto/ap_card.c |  6 ++++++
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/drivers/s390/crypto/ap_bus.c b/drivers/s390/crypto/ap_bus.c
index d82df5b4e2db..639a02f41cea 100644
--- a/drivers/s390/crypto/ap_bus.c
+++ b/drivers/s390/crypto/ap_bus.c
@@ -76,6 +76,10 @@ MODULE_PARM_DESC(useirq, "Use interrupt if available, default is 1 (on).");
 atomic_t ap_max_msg_size = ATOMIC_INIT(AP_DEFAULT_MAX_MSG_SIZE);
 EXPORT_SYMBOL(ap_max_msg_size);
 
+unsigned int ap_max_msg_size_limit = AP_DEFAULT_MAX_MSG_SIZE_LIMIT;
+module_param_named(max_msg_size_limit, ap_max_msg_size_limit, uint, 0444);
+MODULE_PARM_DESC(max_msg_size_limit, "Upper limit for AP max message size.");
+
 static struct device *ap_root_device;
 
 /* Hashtable of all queue devices on the AP bus */
@@ -2631,6 +2635,13 @@ static int __init ap_module_init(void)
 		ap_domain_index = -1;
 	}
 
+	/* verify AP max msg size limit module parameter */
+	if (ap_max_msg_size_limit < AP_MIN_MSG_SIZE) {
+		pr_warn("max_msg_size_limit (%u) below minimum %u, adjusting\n",
+			ap_max_msg_size_limit, AP_MIN_MSG_SIZE);
+		ap_max_msg_size_limit = AP_MIN_MSG_SIZE;
+	}
+
 	/* Create /sys/bus/ap. */
 	rc = bus_register(&ap_bus_type);
 	if (rc)
diff --git a/drivers/s390/crypto/ap_bus.h b/drivers/s390/crypto/ap_bus.h
index 34f9dcd96bc7..5dcc9c54e32e 100644
--- a/drivers/s390/crypto/ap_bus.h
+++ b/drivers/s390/crypto/ap_bus.h
@@ -25,11 +25,14 @@
 #define AP_RESET_TIMEOUT (HZ*0.7)	/* Time in ticks for reset timeouts. */
 #define AP_CONFIG_TIME 30	/* Time in seconds between AP bus rescans. */
 #define AP_POLL_TIME 1		/* Time in ticks between receive polls. */
-#define AP_DEFAULT_MAX_MSG_SIZE (12 * 1024)
+#define AP_MIN_MSG_SIZE (12 * 1024)
+#define AP_DEFAULT_MAX_MSG_SIZE AP_MIN_MSG_SIZE
+#define AP_DEFAULT_MAX_MSG_SIZE_LIMIT (72 * 1024)
 #define AP_TAPQ_ML_FIELD_CHUNK_SIZE (4096)
 
 extern int ap_domain_index;
 extern atomic_t ap_max_msg_size;
+extern unsigned int ap_max_msg_size_limit;
 
 extern DECLARE_HASHTABLE(ap_queues, 8);
 extern spinlock_t ap_queues_lock;
diff --git a/drivers/s390/crypto/ap_card.c b/drivers/s390/crypto/ap_card.c
index e9a8180f08c2..134c14fb2e03 100644
--- a/drivers/s390/crypto/ap_card.c
+++ b/drivers/s390/crypto/ap_card.c
@@ -14,6 +14,7 @@
 #include <asm/sclp.h>
 
 #include "ap_bus.h"
+#include "ap_debug.h"
 
 /*
  * AP card related attributes.
@@ -244,6 +245,11 @@ struct ap_card *ap_card_create(int id, struct ap_tapq_hwinfo hwinfo,
 	ac->maxmsgsize = hwinfo.ml > 3 ?
 		hwinfo.ml * (hwinfo.mlm + 1) * AP_TAPQ_ML_FIELD_CHUNK_SIZE :
 		AP_DEFAULT_MAX_MSG_SIZE;
+	if (ac->maxmsgsize > ap_max_msg_size_limit) {
+		AP_DBF_WARN("card 0x%02x: ml=%d mlm=%d exceeds max_msg_size_limit %u, clipping\n",
+			    id, hwinfo.ml, hwinfo.mlm, ap_max_msg_size_limit);
+		ac->maxmsgsize = ap_max_msg_size_limit;
+	}
 
 	return ac;
 }
-- 
2.43.0