[PATCH v2 2/2] riscv: Set SHMLBA according to cache geometry

[Marc Gauthier <consult-mg@gstardust.com>]

Set SHMLBA to the maximum cache "span" (line size * number of sets) of
all CPU L1 instruction and data caches (L2 and up are rarely VIPT).
This avoids VIPT cache aliasing with minimal alignment constraints.

If the device tree does not provide cache parameters, use a conservative
default of 16 KB:  only large enough to avoid aliasing in most VIPT caches.

Signed-off-by: Marc Gauthier <consult-mg@gstardust.com>
---
 arch/riscv/include/asm/Kbuild     |  1 -
 arch/riscv/include/asm/shmparam.h | 12 +++++++
 arch/riscv/kernel/cacheinfo.c     | 52 +++++++++++++++++++++++++++++++
 3 files changed, 64 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/include/asm/shmparam.h

diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
index 16970f246860..3905765807af 100644
--- a/arch/riscv/include/asm/Kbuild
+++ b/arch/riscv/include/asm/Kbuild
@@ -27,7 +27,6 @@ generic-y += percpu.h
 generic-y += preempt.h
 generic-y += sections.h
 generic-y += serial.h
-generic-y += shmparam.h
 generic-y += topology.h
 generic-y += trace_clock.h
 generic-y += unaligned.h
diff --git a/arch/riscv/include/asm/shmparam.h b/arch/riscv/include/asm/shmparam.h
new file mode 100644
index 000000000000..9b6a98153648
--- /dev/null
+++ b/arch/riscv/include/asm/shmparam.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_RISCV_SHMPARAM_H
+#define _ASM_RISCV_SHMPARAM_H
+
+/*
+ * Minimum alignment of shared memory segments as a function of cache geometry.
+ */
+#define	SHMLBA	arch_shmlba()
+
+long arch_shmlba(void);
+
+#endif /* _ASM_RISCV_SHMPARAM_H */
diff --git a/arch/riscv/kernel/cacheinfo.c b/arch/riscv/kernel/cacheinfo.c
index 4c90c07d8c39..1bc7df8577d6 100644
--- a/arch/riscv/kernel/cacheinfo.c
+++ b/arch/riscv/kernel/cacheinfo.c
@@ -1,12 +1,61 @@
 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * Copyright (C) 2017 SiFive
+ * Copyright (C) 2019 Aril Inc
  */
 
 #include <linux/cacheinfo.h>
 #include <linux/cpu.h>
 #include <linux/of.h>
 #include <linux/of_device.h>
+#include <linux/mm.h>
+
+static long shmlba;
+
+
+/*
+ * Assuming  cache size = line size * #sets * N  for N-way associative caches,
+ * return the max cache "span" == (line size * #sets) == (cache size / N)
+ * across all L1 caches, or 0 if cache parameters are not available.
+ * VIPT caches with span > min page size are susceptible to aliasing.
+ */
+static long get_max_cache_span(void)
+{
+	struct cpu_cacheinfo *this_cpu_ci;
+	struct cacheinfo *this_leaf;
+	long span, max_span = 0;
+	int cpu, leaf;
+
+	for_each_possible_cpu(cpu) {
+		this_cpu_ci = get_cpu_cacheinfo(cpu);
+		this_leaf = this_cpu_ci->info_list;
+		for (leaf = 0; leaf < this_cpu_ci->num_leaves; leaf++) {
+			if (this_leaf->level > 1)
+				break;
+			span = this_leaf->coherency_line_size *
+			       this_leaf->number_of_sets;
+			if (span > max_span)
+				max_span = span;
+			this_leaf++;
+		}
+	}
+	return max_span;
+}
+
+/*
+ * Align shared mappings to the maximum cache "span" to avoid aliasing
+ * in VIPT caches, for performance.
+ * The returned SHMLBA value is always a power-of-two multiple of PAGE_SIZE.
+ */
+long arch_shmlba(void)
+{
+	if (shmlba == 0) {
+		long max_span = get_max_cache_span();
+
+		shmlba = max_span ? PAGE_ALIGN(max_span) : 4 * PAGE_SIZE;
+	}
+	return shmlba;
+}
 
 static void ci_leaf_init(struct cacheinfo *this_leaf,
 			 struct device_node *node,
@@ -93,6 +142,9 @@ static int __populate_cache_leaves(unsigned int cpu)
 	}
 	of_node_put(np);
 
+	/* Force recalculating SHMLBA if cache parameters are updated. */
+	shmlba = 0;
+
 	return 0;
 }
 
-- 
2.17.1


_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv