kaslr: rename physmem_end and PHYSMEM_END to direct_map_physmem_end

For clarity.  It's increasingly hard to reason about the code, when KASLR
is moving around the boundaries.  In this case where KASLR is randomizing
the location of the kernel image within physical memory, the maximum
number of address bits for physical memory has not changed.

What has changed is the ending address of memory that is allowed to be
directly mapped by the kernel.

Let's name the variable, and the associated macro accordingly.

Also, enhance the comment above the direct_map_physmem_end definition,
to further clarify how this all works.

Link: https://lkml.kernel.org/r/20241009025024.89813-1-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Jordan Niethe <jniethe@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 9 insertions, 5 deletions

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 230f1dee4f09..70d3353c92fa 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -52,7 +52,7 @@ static __initdata struct kaslr_memory_region {
 } kaslr_regions[] = {
 	{
 		.base	= &page_offset_base,
-		.end	= &physmem_end,
+		.end	= &direct_map_physmem_end,
 	},
 	{
 		.base	= &vmalloc_base,
@@ -62,8 +62,12 @@ static __initdata struct kaslr_memory_region {
 	},
 };
 
-/* The end of the possible address space for physical memory */
-unsigned long physmem_end __ro_after_init;
+/*
+ * The end of the physical address space that can be mapped directly by the
+ * kernel. This starts out at (1<<MAX_PHYSMEM_BITS) - 1), but KASLR may reduce
+ * that in order to increase the available entropy for mapping other regions.
+ */
+unsigned long direct_map_physmem_end __ro_after_init;
 
 /* Get size in bytes used by the memory region */
 static inline unsigned long get_padding(struct kaslr_memory_region *region)
@@ -94,7 +98,7 @@ void __init kernel_randomize_memory(void)
 	BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);
 
 	/* Preset the end of the possible address space for physical memory */
-	physmem_end = ((1ULL << MAX_PHYSMEM_BITS) - 1);
+	direct_map_physmem_end = ((1ULL << MAX_PHYSMEM_BITS) - 1);
 	if (!kaslr_memory_enabled())
 		return;
 
@@ -145,7 +149,7 @@ void __init kernel_randomize_memory(void)
 		vaddr += get_padding(&kaslr_regions[i]);
 		/*
 		 * KASLR trims the maximum possible size of the
-		 * direct-map. Update the physmem_end boundary.
+		 * direct-map. Update the direct_map_physmem_end boundary.
 		 * No rounding required as the region starts
 		 * PUD aligned and size is in units of TB.
 		 */