arm64: mm: use single quantity to represent the PA to VA translation

On arm64, the global variable memstart_addr represents the physical
address of PAGE_OFFSET, and so physical to virtual translations or
vice versa used to come down to simple additions or subtractions
involving the values of PAGE_OFFSET and memstart_addr.

When support for 52-bit virtual addressing was introduced, we had to
deal with PAGE_OFFSET potentially being outside of the region that
can be covered by the virtual range (as the 52-bit VA capable build
needs to be able to run on systems that are only 48-bit VA capable),
and for this reason, another translation was introduced, and recorded
in the global variable physvirt_offset.

However, if we go back to the original definition of memstart_addr,
i.e., the physical address of PAGE_OFFSET, it turns out that there is
no need for two separate translations: instead, we can simply subtract
the size of the unaddressable VA space from memstart_addr to make the
available physical memory appear in the 48-bit addressable VA region.

This simplifies things, but also fixes a bug on KASLR builds, which
may update memstart_addr later on in arm64_memblock_init(), but fails
to update vmemmap and physvirt_offset accordingly.

Fixes: 5383cc6 ("arm64: mm: Introduce vabits_actual")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Link: https://lore.kernel.org/r/20201008153602.9467-2-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/memory.h

@@ -169,7 +169,6 @@
 extern u64			vabits_actual;
 #define PAGE_END		(_PAGE_END(vabits_actual))
 
-extern s64			physvirt_offset;
 extern s64			memstart_addr;
 /* PHYS_OFFSET - the physical address of the start of memory. */
 #define PHYS_OFFSET		({ VM_BUG_ON(memstart_addr & 1); memstart_addr; })
@@ -245,7 +244,7 @@ static inline const void *__tag_set(const void *addr, u8 tag)
  */
 #define __is_lm_address(addr)	(!(((u64)addr) & BIT(vabits_actual - 1)))
 
-#define __lm_to_phys(addr)	(((addr) + physvirt_offset))
+#define __lm_to_phys(addr)	(((addr) & ~PAGE_OFFSET) + PHYS_OFFSET)
 #define __kimg_to_phys(addr)	((addr) - kimage_voffset)
 
 #define __virt_to_phys_nodebug(x) ({					\
@@ -263,7 +262,7 @@ extern phys_addr_t __phys_addr_symbol(unsigned long x);
 #define __phys_addr_symbol(x)	__pa_symbol_nodebug(x)
 #endif /* CONFIG_DEBUG_VIRTUAL */
 
-#define __phys_to_virt(x)	((unsigned long)((x) - physvirt_offset))
+#define __phys_to_virt(x)	((unsigned long)((x) - PHYS_OFFSET) | PAGE_OFFSET)
 #define __phys_to_kimg(x)	((unsigned long)((x) + kimage_voffset))
 
 /*

arch/arm64/include/asm/pgtable.h

@@ -24,6 +24,8 @@
 #define VMALLOC_START		(MODULES_END)
 #define VMALLOC_END		(- PUD_SIZE - VMEMMAP_SIZE - SZ_64K)
 
+#define vmemmap			((struct page *)VMEMMAP_START - (memstart_addr >> PAGE_SHIFT))
+
 #define FIRST_USER_ADDRESS	0UL
 
 #ifndef __ASSEMBLY__
@@ -34,8 +36,6 @@
 #include <linux/mm_types.h>
 #include <linux/sched.h>
 
-extern struct page *vmemmap;
-
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 #define __HAVE_ARCH_FLUSH_PMD_TLB_RANGE
 

arch/arm64/mm/init.c

@@ -54,12 +54,6 @@
 s64 memstart_addr __ro_after_init = -1;
 EXPORT_SYMBOL(memstart_addr);
 
-s64 physvirt_offset __ro_after_init;
-EXPORT_SYMBOL(physvirt_offset);
-
-struct page *vmemmap __ro_after_init;
-EXPORT_SYMBOL(vmemmap);
-
 /*
  * We create both ZONE_DMA and ZONE_DMA32. ZONE_DMA covers the first 1G of
  * memory as some devices, namely the Raspberry Pi 4, have peripherals with
@@ -290,20 +284,6 @@ void __init arm64_memblock_init(void)
 	memstart_addr = round_down(memblock_start_of_DRAM(),
 				   ARM64_MEMSTART_ALIGN);
 
-	physvirt_offset = PHYS_OFFSET - PAGE_OFFSET;
-
-	vmemmap = ((struct page *)VMEMMAP_START - (memstart_addr >> PAGE_SHIFT));
-
-	/*
-	 * If we are running with a 52-bit kernel VA config on a system that
-	 * does not support it, we have to offset our vmemmap and physvirt_offset
-	 * s.t. we avoid the 52-bit portion of the direct linear map
-	 */
-	if (IS_ENABLED(CONFIG_ARM64_VA_BITS_52) && (vabits_actual != 52)) {
-		vmemmap += (_PAGE_OFFSET(48) - _PAGE_OFFSET(52)) >> PAGE_SHIFT;
-		physvirt_offset = PHYS_OFFSET - _PAGE_OFFSET(48);
-	}
-
 	/*
 	 * Remove the memory that we will not be able to cover with the
 	 * linear mapping. Take care not to clip the kernel which may be
@@ -318,6 +298,16 @@ void __init arm64_memblock_init(void)
 		memblock_remove(0, memstart_addr);
 	}
 
+	/*
+	 * If we are running with a 52-bit kernel VA config on a system that
+	 * does not support it, we have to place the available physical
+	 * memory in the 48-bit addressable part of the linear region, i.e.,
+	 * we have to move it upward. Since memstart_addr represents the
+	 * physical address of PAGE_OFFSET, we have to *subtract* from it.
+	 */
+	if (IS_ENABLED(CONFIG_ARM64_VA_BITS_52) && (vabits_actual != 52))
+		memstart_addr -= _PAGE_OFFSET(48) - _PAGE_OFFSET(52);
+
 	/*
 	 * Apply the memory limit if it was set. Since the kernel may be loaded
 	 * high up in memory, add back the kernel region that must be accessible