[PATCH] powerpc/powernv: utilising darn instruction for get_random_seed on p9

[Matt Brown <matthew.brown.dev@gmail.com>]

Currently ppc_md.get_random_seed uses the powernv_get_random_long function.
A guest calling this function would have to go through the hypervisor. The
'darn' instruction, introduced in power 9, allows us to bypass this by
directly obtaining a value from the mmio region.

This patch adds an alternative function for ppc_md.get_random_seed on p9,
utilising the darn instruction.

Signed-off-by: Matt Brown <matthew.brown.dev@gmail.com>
---
 arch/powerpc/include/asm/ppc-opcode.h |  4 ++++
 arch/powerpc/platforms/powernv/rng.c  | 23 ++++++++++++++++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/ppc-opcode.h b/arch/powerpc/include/asm/ppc-opcode.h
index c4ced1d..d5f7082 100644
--- a/arch/powerpc/include/asm/ppc-opcode.h
+++ b/arch/powerpc/include/asm/ppc-opcode.h
@@ -134,6 +134,7 @@
 #define PPC_INST_COPY			0x7c00060c
 #define PPC_INST_COPY_FIRST		0x7c20060c
 #define PPC_INST_CP_ABORT		0x7c00068c
+#define PPC_INST_DARN			0x7c0005e6
 #define PPC_INST_DCBA			0x7c0005ec
 #define PPC_INST_DCBA_MASK		0xfc0007fe
 #define PPC_INST_DCBAL			0x7c2005ec
@@ -325,6 +326,9 @@
 
 /* Deal with instructions that older assemblers aren't aware of */
 #define	PPC_CP_ABORT		stringify_in_c(.long PPC_INST_CP_ABORT)
+#define PPC_DARN(t, l)		stringify_in_c(.long PPC_INST_DARN |  \
+						___PPC_RT(t)	   |  \
+						___PPC_RA(l))
 #define	PPC_DCBAL(a, b)		stringify_in_c(.long PPC_INST_DCBAL | \
 					__PPC_RA(a) | __PPC_RB(b))
 #define	PPC_DCBZL(a, b)		stringify_in_c(.long PPC_INST_DCBZL | \
diff --git a/arch/powerpc/platforms/powernv/rng.c b/arch/powerpc/platforms/powernv/rng.c
index 5dcbdea..db34f32 100644
--- a/arch/powerpc/platforms/powernv/rng.c
+++ b/arch/powerpc/platforms/powernv/rng.c
@@ -8,6 +8,7 @@
  */
 
 #define pr_fmt(fmt)	"powernv-rng: " fmt
+#define DARN_ERR 0xFFFFFFFFFFFFFFFFul
 
 #include <linux/kernel.h>
 #include <linux/of.h>
@@ -16,6 +17,7 @@
 #include <linux/slab.h>
 #include <linux/smp.h>
 #include <asm/archrandom.h>
+#include <asm/cputable.h>
 #include <asm/io.h>
 #include <asm/prom.h>
 #include <asm/machdep.h>
@@ -67,6 +69,22 @@ int powernv_get_random_real_mode(unsigned long *v)
 	return 1;
 }
 
+int powernv_get_random_darn(unsigned long *v)
+{
+	int n = 0;
+	unsigned long val;
+
+	do {
+		/* Using DARN with L=1 - conditioned random number */
+		asm (PPC_DARN(%0, 1)"\n" : "=r"(val) :);
+		n++;
+	} while (val == DARN_ERR && n < 10);
+
+	*v = val;
+
+	return (val == DARN_ERR) ? 0 : 1;
+}
+
 int powernv_get_random_long(unsigned long *v)
 {
 	struct powernv_rng *rng;
@@ -128,7 +146,10 @@ static __init int rng_create(struct device_node *dn)
 
 	pr_info_once("Registering arch random hook.\n");
 
-	ppc_md.get_random_seed = powernv_get_random_long;
+	if (cpu_has_feature(CPU_FTR_ARCH_300))
+		ppc_md.get_random_seed = powernv_get_random_darn;
+	else
+		ppc_md.get_random_seed = powernv_get_random_long;
 
 	return 0;
 }
-- 
2.9.3