[PATCH 3/3] ARM: Map the lowmem and kernel separately

[Linus Walleij <linus.walleij@linaro.org>]

Using our knowledge of where the physical kernel sections start
and end we can split mapping of lowmem and kernel apart.

This is helpful when you want to place the kernel independently
from lowmem and not be limited to putting it into lowmem only,
but also into places such as the VMALLOC area.

We extensively rewrite the lowmem mapping code to account for
all cases where the kernel image overlaps with the lowmem in
different ways. This is helpful to handle situations which
occur when the kernel is loaded in different places and makes
it possible to place the kernel in a more random manner
which is done with e.g. KASLR.

We sprinkle some comments with illustrations and pr_debug()
over it so it is also very evident to readers what is happening.

We now use the kernel_sec_start and kernel_sec_end instead
of relying on __pa() (phys_to_virt) to provide this. This
is helpful if we want to resolve physical-to-virtual and
virtual-to-physical mappings at runtime rather than
compiletime, especially if we are not using patch phys to
virt.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 arch/arm/mm/mmu.c | 144 +++++++++++++++++++++++++++++++++++-----------
 1 file changed, 111 insertions(+), 33 deletions(-)

diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index c1e12aab67b8..ac37959507b7 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -1457,8 +1457,6 @@ static void __init kmap_init(void)
 
 static void __init map_lowmem(void)
 {
-	phys_addr_t kernel_x_start = round_down(__pa(KERNEL_START), SECTION_SIZE);
-	phys_addr_t kernel_x_end = round_up(__pa(__init_end), SECTION_SIZE);
 	phys_addr_t start, end;
 	u64 i;
 
@@ -1466,55 +1464,126 @@ static void __init map_lowmem(void)
 	for_each_mem_range(i, &start, &end) {
 		struct map_desc map;
 
+		pr_debug("map lowmem start: 0x%08llx, end: 0x%08llx\n",
+			 (long long)start, (long long)end);
 		if (end > arm_lowmem_limit)
 			end = arm_lowmem_limit;
 		if (start >= end)
 			break;
 
-		if (end < kernel_x_start) {
-			map.pfn = __phys_to_pfn(start);
-			map.virtual = __phys_to_virt(start);
-			map.length = end - start;
-			map.type = MT_MEMORY_RWX;
+		/*
+		 * If our kernel image is in the VMALLOC area we need to remove
+		 * the kernel physical memory from lowmem since the kernel will
+		 * be mapped separately.
+		 *
+		 * The kernel will typically be at the very start of lowmem,
+		 * but any placement relative to memory ranges is possible.
+		 *
+		 * If the memblock contains the kernel, we have to chisel out
+		 * the kernel memory from it and map each part separately. We
+		 * get 6 different theoretical cases:
+		 *
+		 *                            +--------+ +--------+
+		 *  +-- start --+  +--------+ | Kernel | | Kernel |
+		 *  |           |  | Kernel | | case 2 | | case 5 |
+		 *  |           |  | case 1 | +--------+ |        | +--------+
+		 *  |  Memory   |  +--------+            |        | | Kernel |
+		 *  |  range    |  +--------+            |        | | case 6 |
+		 *  |           |  | Kernel | +--------+ |        | +--------+
+		 *  |           |  | case 3 | | Kernel | |        |
+		 *  +-- end ----+  +--------+ | case 4 | |        |
+		 *                            +--------+ +--------+
+		 */
 
-			create_mapping(&map);
-		} else if (start >= kernel_x_end) {
-			map.pfn = __phys_to_pfn(start);
-			map.virtual = __phys_to_virt(start);
-			map.length = end - start;
-			map.type = MT_MEMORY_RW;
+		/* Case 5: kernel covers range, don't map anything, should be rare */
+		if ((start > kernel_sec_start) && (end < kernel_sec_end))
+			break;
 
-			create_mapping(&map);
-		} else {
-			/* This better cover the entire kernel */
-			if (start < kernel_x_start) {
+		/* Cases where the kernel is starting inside the range */
+		if ((kernel_sec_start >= start) && (kernel_sec_start <= end)) {
+			/* Case 6: kernel is embedded in the range, we need two mappings */
+			if ((start < kernel_sec_start) && (end > kernel_sec_end)) {
+				/* Map memory below the kernel */
 				map.pfn = __phys_to_pfn(start);
 				map.virtual = __phys_to_virt(start);
-				map.length = kernel_x_start - start;
+				map.length = kernel_sec_start - start;
 				map.type = MT_MEMORY_RW;
-
 				create_mapping(&map);
-			}
-
-			map.pfn = __phys_to_pfn(kernel_x_start);
-			map.virtual = __phys_to_virt(kernel_x_start);
-			map.length = kernel_x_end - kernel_x_start;
-			map.type = MT_MEMORY_RWX;
-
-			create_mapping(&map);
-
-			if (kernel_x_end < end) {
-				map.pfn = __phys_to_pfn(kernel_x_end);
-				map.virtual = __phys_to_virt(kernel_x_end);
-				map.length = end - kernel_x_end;
+				/* Map memory above the kernel */
+				map.pfn = __phys_to_pfn(kernel_sec_end);
+				map.virtual = __phys_to_virt(kernel_sec_end);
+				map.length = end - kernel_sec_end;
 				map.type = MT_MEMORY_RW;
-
 				create_mapping(&map);
+				break;
 			}
+			/* Case 1: kernel and range start at the same address, should be common */
+			if (kernel_sec_start == start)
+				start = kernel_sec_end;
+			/* Case 3: kernel and range end at the same address, should be rare */
+			if (kernel_sec_end == end)
+				end = kernel_sec_start;
+		} else if ((kernel_sec_start < start) && (kernel_sec_end > start) && (kernel_sec_end < end)) {
+			/* Case 2: kernel ends inside range, starts below it */
+			start = kernel_sec_end;
+		} else if ((kernel_sec_start > start) && (kernel_sec_start < end) && (kernel_sec_end > end)) {
+			/* Case 4: kernel starts inside range, ends above it */
+			end = kernel_sec_start;
 		}
+		map.pfn = __phys_to_pfn(start);
+		map.virtual = __phys_to_virt(start);
+		map.length = end - start;
+		map.type = MT_MEMORY_RW;
+		create_mapping(&map);
 	}
 }
 
+static void __init map_kernel(void)
+{
+	/*
+	 * We use the well known kernel section start and end and split the area in the
+	 * middle like this:
+	 *  .                .
+	 *  | RW memory      |
+	 *  +----------------+ kernel_x_start
+	 *  | Executable     |
+	 *  | kernel memory  |
+	 *  +----------------+ kernel_x_end / kernel_nx_start
+	 *  | Non-executable |
+	 *  | kernel memory  |
+	 *  +----------------+ kernel_nx_end
+	 *  | RW memory      |
+	 *  .                .
+	 *
+	 * Notice that we are dealing with section sized mappings here so all of this
+	 * will be bumped to the closest section boundary. This means that some of the
+	 * non-executable part of the kernel memory is actually mapped as executable.
+	 * This will only persist until we turn on proper memory management later on
+	 * and we remap the whole kernel with page granularity.
+	 */
+	phys_addr_t kernel_x_start = kernel_sec_start;
+	phys_addr_t kernel_x_end = round_up(__pa(__init_end), SECTION_SIZE);
+	phys_addr_t kernel_nx_start = kernel_x_end;
+	phys_addr_t kernel_nx_end = kernel_sec_end;
+	struct map_desc map;
+
+	map.pfn = __phys_to_pfn(kernel_x_start);
+	map.virtual = __phys_to_virt(kernel_x_start);
+	map.length = kernel_x_end - kernel_x_start;
+	map.type = MT_MEMORY_RWX;
+	create_mapping(&map);
+
+	/* If the nx part is small it may end up covered by the tail of the RWX section */
+	if (kernel_x_end == kernel_nx_end)
+		return;
+
+	map.pfn = __phys_to_pfn(kernel_nx_start);
+	map.virtual = __phys_to_virt(kernel_nx_start);
+	map.length = kernel_nx_end - kernel_nx_start;
+	map.type = MT_MEMORY_RW;
+	create_mapping(&map);
+}
+
 #ifdef CONFIG_ARM_PV_FIXUP
 typedef void pgtables_remap(long long offset, unsigned long pgd);
 pgtables_remap lpae_pgtables_remap_asm;
@@ -1645,9 +1714,18 @@ void __init paging_init(const struct machine_desc *mdesc)
 {
 	void *zero_page;
 
+	pr_debug("physical kernel sections: 0x%08x-0x%08x\n",
+		 kernel_sec_start, kernel_sec_end);
+
 	prepare_page_table();
 	map_lowmem();
 	memblock_set_current_limit(arm_lowmem_limit);
+	pr_debug("lowmem limit is %08llx\n", (long long)arm_lowmem_limit);
+	/*
+	 * After this point early_alloc(), i.e. the memblock allocator, can
+	 * be used
+	 */
+	map_kernel();
 	dma_contiguous_remap();
 	early_fixmap_shutdown();
 	devicemaps_init(mdesc);
-- 
2.31.1


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