arm64: mm: Move fixmap region above vmemmap region

Move the fixmap region above the vmemmap region, so that the start of
the vmemmap delineates the end of the region available for vmalloc and
vmap allocations and the randomized placement of the kernel and modules.

In a subsequent patch, we will take advantage of this to reclaim most of
the vmemmap area when running a 52-bit VA capable build with 52-bit
virtual addressing disabled at runtime.

Note that the existing guard region of 256 MiB covers the fixmap and PCI
I/O regions as well, so we can reduce it 8 MiB, which is what we use in
other places too.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20231213084024.2367360-11-ardb@google.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>

arch/arm64/include/asm/memory.h

@@ -51,7 +51,7 @@
 #define VMEMMAP_END		(VMEMMAP_START + VMEMMAP_SIZE)
 #define PCI_IO_START		(VMEMMAP_END + SZ_8M)
 #define PCI_IO_END		(PCI_IO_START + PCI_IO_SIZE)
-#define FIXADDR_TOP		(VMEMMAP_START - SZ_32M)
+#define FIXADDR_TOP		(-UL(SZ_8M))
 
 #if VA_BITS > 48
 #define VA_BITS_MIN		(48)

arch/arm64/include/asm/pgtable.h

@@ -22,7 +22,7 @@
  *	and fixed mappings
  */
 #define VMALLOC_START		(MODULES_END)
-#define VMALLOC_END		(VMEMMAP_START - SZ_256M)
+#define VMALLOC_END		(VMEMMAP_START - SZ_8M)
 
 #define vmemmap			((struct page *)VMEMMAP_START - (memstart_addr >> PAGE_SHIFT))
 

arch/arm64/mm/fixmap.c

@@ -16,6 +16,9 @@
 #include <asm/pgalloc.h>
 #include <asm/tlbflush.h>
 
+/* ensure that the fixmap region does not grow down into the PCI I/O region */
+static_assert(FIXADDR_TOT_START > PCI_IO_END);
+
 #define NR_BM_PTE_TABLES \
 	SPAN_NR_ENTRIES(FIXADDR_TOT_START, FIXADDR_TOP, PMD_SHIFT)
 #define NR_BM_PMD_TABLES \

arch/arm64/mm/ptdump.c

@@ -45,12 +45,12 @@ static struct addr_marker address_markers[] = {
 	{ MODULES_END,			"Modules end" },
 	{ VMALLOC_START,		"vmalloc() area" },
 	{ VMALLOC_END,			"vmalloc() end" },
-	{ FIXADDR_TOT_START,		"Fixmap start" },
-	{ FIXADDR_TOP,			"Fixmap end" },
 	{ VMEMMAP_START,		"vmemmap start" },
 	{ VMEMMAP_END,			"vmemmap end" },
 	{ PCI_IO_START,			"PCI I/O start" },
 	{ PCI_IO_END,			"PCI I/O end" },
+	{ FIXADDR_TOT_START,		"Fixmap start" },
+	{ FIXADDR_TOP,			"Fixmap end" },
 	{ -1,				NULL },
 };