[PATCH 0/2] riscv: Align shared mappings to avoid cache aliasing

[PATCH 0/2] riscv: Align shared mappings to avoid cache aliasing

[Marc Gauthier]

Ensure optimal performance on larger VIPT caches by aligning shared
mappings to the maximum cache "span" (line size * number of sets) of
all CPU L1 instruction and data caches (L2 and up are rarely VIPT).
If the device tree does not provide cache parameters, use a conservative
16 KB alignment:  only large enough to avoid aliasing in most VIPT caches.

Tested on Linux 5.4-rc7 in simulation.

Signed-off-by: Marc Gauthier <consult-mg@gstardust.com>

Marc Gauthier (2):
  riscv: Align shared mappings to SHMLBA
  riscv: Set SHMLBA according to cache geometry

 arch/riscv/include/asm/pgtable.h  |   4 ++
 arch/riscv/include/asm/shmparam.h |  12 ++++
 arch/riscv/kernel/cacheinfo.c     |  49 +++++++++++++
 arch/riscv/kernel/sys_riscv.c     | 112 ++++++++++++++++++++++++++++++
 4 files changed, 177 insertions(+)
 create mode 100644 arch/riscv/include/asm/shmparam.h

-- 
2.17.1


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[PATCH 1/2] riscv: Align shared mappings to SHMLBA

[Marc Gauthier]

Align shared mappings according to SHMLBA for VIPT cache performance.

arch_get_unmapped_area() and arch_get_unmapped_area_topdown() are
essentially copies of their default implementations in mm/mmap.c,
modified to align the address to SHMLBA for shared mappings, i.e.
where MAP_SHARED is specified or a file pointer is provided.

Allow MAP_FIXED to request unaligned shared mappings.  Although this
may potentially reduce performance, very little software does this, as
it is not portable across architectures that enforce alignment.

Signed-off-by: Marc Gauthier <consult-mg@gstardust.com>
---
 arch/riscv/include/asm/pgtable.h |   4 ++
 arch/riscv/kernel/sys_riscv.c    | 112 +++++++++++++++++++++++++++++++
 2 files changed, 116 insertions(+)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index d3221017194d..7d1cc47ac5f9 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -436,6 +436,10 @@ extern void *dtb_early_va;
 extern void setup_bootmem(void);
 extern void paging_init(void);
 
+/* We provide arch_get_unmapped_area to handle VIPT caches efficiently. */
+#define HAVE_ARCH_UNMAPPED_AREA
+#define HAVE_ARCH_UNMAPPED_AREA_TOPDOWN
+
 /*
  * Task size is 0x4000000000 for RV64 or 0x9fc00000 for RV32.
  * Note that PGDIR_SIZE must evenly divide TASK_SIZE.
diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
index f3619f59d85c..3465811f718e 100644
--- a/arch/riscv/kernel/sys_riscv.c
+++ b/arch/riscv/kernel/sys_riscv.c
@@ -3,11 +3,15 @@
  * Copyright (C) 2012 Regents of the University of California
  * Copyright (C) 2014 Darius Rad <darius@bluespec.com>
  * Copyright (C) 2017 SiFive
+ * Copyright (C) 2019 Aril Inc
  */
 
 #include <linux/syscalls.h>
 #include <asm/unistd.h>
 #include <asm/cacheflush.h>
+#include <linux/shm.h>
+#include <linux/mman.h>
+#include <linux/security.h>
 
 static long riscv_sys_mmap(unsigned long addr, unsigned long len,
 			   unsigned long prot, unsigned long flags,
@@ -65,3 +69,111 @@ SYSCALL_DEFINE3(riscv_flush_icache, uintptr_t, start, uintptr_t, end,
 
 	return 0;
 }
+
+/*
+ * RISC-V requires implementations to function correctly in the presence
+ * of cache aliasing, regardless of page alignment.  It says nothing about
+ * performance.  To ensure healthy performance with commonly implemented
+ * VIPT caches, the following code avoids most cases of cache aliasing by
+ * aligning shared mappings such that all mappings of a given physical
+ * page of an object are at a multiple of SHMLBA bytes from each other.
+ *
+ * It does not enforce alignment.  Using MAP_FIXED to request unaligned
+ * shared mappings is not common, and may perform poorly with VIPT caches.
+ */
+unsigned long
+arch_get_unmapped_area(struct file *filp, unsigned long addr,
+                unsigned long len, unsigned long pgoff, unsigned long flags)
+{
+	struct mm_struct *mm = current->mm;
+	struct vm_area_struct *vma, *prev;
+	struct vm_unmapped_area_info info;
+	const unsigned long pgoffset = pgoff << PAGE_SHIFT;
+	int do_align = (filp || (flags & MAP_SHARED));
+
+	if (len > TASK_SIZE - mmap_min_addr)
+		return -ENOMEM;
+
+	if (flags & MAP_FIXED)
+		return addr;
+
+	if (addr) {
+		if (do_align)
+			addr = ALIGN(addr, SHMLBA) + (pgoffset & (SHMLBA - 1));
+		else
+			addr = PAGE_ALIGN(addr);
+		vma = find_vma_prev(mm, addr, &prev);
+		if (TASK_SIZE - len >= addr && addr >= mmap_min_addr &&
+		    (!vma || addr + len <= vm_start_gap(vma)) &&
+		    (!prev || addr >= vm_end_gap(prev)))
+			return addr;
+	}
+
+	info.flags = 0;
+	info.length = len;
+	info.low_limit = mm->mmap_base;
+	info.high_limit = TASK_SIZE;
+	info.align_mask = do_align ? SHMLBA - 1 : 0;
+	info.align_offset = pgoffset;
+	return vm_unmapped_area(&info);
+}
+
+/*
+ * Similar to arch_get_unmapped_area(), but allocating top-down from below the
+ * stack's low limit (the base).
+ */
+unsigned long
+arch_get_unmapped_area_topdown(struct file *filp, unsigned long addr,
+			  unsigned long len, unsigned long pgoff,
+			  unsigned long flags)
+{
+	struct vm_area_struct *vma, *prev;
+	struct mm_struct *mm = current->mm;
+	struct vm_unmapped_area_info info;
+	const unsigned long pgoffset = pgoff << PAGE_SHIFT;
+	int do_align = (filp || (flags & MAP_SHARED));
+
+	/* requested length too big for entire address space */
+	if (len > TASK_SIZE - mmap_min_addr)
+		return -ENOMEM;
+
+	if (flags & MAP_FIXED)
+		return addr;
+
+	/* requesting a specific address */
+	if (addr) {
+		if (do_align)
+			addr = ALIGN(addr, SHMLBA) + (pgoffset & (SHMLBA - 1));
+		else
+			addr = PAGE_ALIGN(addr);
+		vma = find_vma_prev(mm, addr, &prev);
+		if (TASK_SIZE - len >= addr && addr >= mmap_min_addr &&
+				(!vma || addr + len <= vm_start_gap(vma)) &&
+				(!prev || addr >= vm_end_gap(prev)))
+			return addr;
+	}
+
+	info.flags = VM_UNMAPPED_AREA_TOPDOWN;
+	info.length = len;
+	info.low_limit = max(PAGE_SIZE, mmap_min_addr);
+	info.high_limit = mm->mmap_base;
+	info.align_mask = do_align ? SHMLBA - 1 : 0;
+	info.align_offset = pgoffset;
+	addr = vm_unmapped_area(&info);
+
+	/*
+	 * A failed mmap() very likely causes application failure,
+	 * so fall back to the bottom-up function here. This scenario
+	 * can happen with large stack limits and large mmap()
+	 * allocations.
+	 */
+	if (offset_in_page(addr)) {
+		VM_BUG_ON(addr != -ENOMEM);
+		info.flags = 0;
+		info.low_limit = TASK_UNMAPPED_BASE;
+		info.high_limit = TASK_SIZE;
+		addr = vm_unmapped_area(&info);
+	}
+
+	return addr;
+}
-- 
2.17.1


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

[Marc Gauthier]

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/shmparam.h | 12 ++++++++
 arch/riscv/kernel/cacheinfo.c     | 49 +++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+)
 create mode 100644 arch/riscv/include/asm/shmparam.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..a7a601ee6a86 100644
--- a/arch/riscv/kernel/cacheinfo.c
+++ b/arch/riscv/kernel/cacheinfo.c
@@ -1,12 +1,58 @@
 // 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++, this_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;
+		}
+	}
+	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 +139,9 @@ 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


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