[RFC PATCH 4/7] riscv: Do not rely on KASAN to define the memory layout

[Samuel Holland]

Commit 66673099f734 ("riscv: mm: Pre-allocate vmemmap/direct map/kasan
PGD entries") used the start of the KASAN shadow memory region to
represent the end of the linear map, since the two memory regions were
immediately adjacent. This is no longer the case for Sv39; commit
5c8405d763dc ("riscv: Extend sv39 linear mapping max size to 128G")
introduced a 4 GiB hole between the regions. Introducing KASAN_SW_TAGS
will cut the size of the shadow memory region in half, creating an even
larger hole.

Avoid wasting PGD entries on this hole by using the size of the linear
map (KERN_VIRT_SIZE) to compute PAGE_END.

Since KASAN_SHADOW_START/KASAN_SHADOW_END are used inside an IS_ENABLED
block, it's not possible to completely hide the constants when KASAN is
disabled, so provide dummy definitions for that case.

Signed-off-by: Samuel Holland <samuel.holland at sifive.com>
---

 arch/riscv/include/asm/kasan.h | 11 +++++++++--
 arch/riscv/mm/init.c           |  2 +-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/include/asm/kasan.h b/arch/riscv/include/asm/kasan.h
index e6a0071bdb56..a4e92ce9fa31 100644
--- a/arch/riscv/include/asm/kasan.h
+++ b/arch/riscv/include/asm/kasan.h
@@ -6,6 +6,8 @@
 
 #ifndef __ASSEMBLY__
 
+#ifdef CONFIG_KASAN
+
 /*
  * The following comment was copied from arm64:
  * KASAN_SHADOW_START: beginning of the kernel virtual addresses.
@@ -33,13 +35,18 @@
 #define KASAN_SHADOW_START	((KASAN_SHADOW_END - KASAN_SHADOW_SIZE) & PGDIR_MASK)
 #define KASAN_SHADOW_END	MODULES_LOWEST_VADDR
 
-#ifdef CONFIG_KASAN
 #define KASAN_SHADOW_OFFSET	_AC(CONFIG_KASAN_SHADOW_OFFSET, UL)
 
 void kasan_init(void);
 asmlinkage void kasan_early_init(void);
 void kasan_swapper_init(void);
 
-#endif
+#else /* CONFIG_KASAN */
+
+#define KASAN_SHADOW_START	MODULES_LOWEST_VADDR
+#define KASAN_SHADOW_END	MODULES_LOWEST_VADDR
+
+#endif /* CONFIG_KASAN */
+
 #endif
 #endif /* __ASM_KASAN_H */
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 8b698d9609e7..1667f1b05f5a 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -1492,7 +1492,7 @@ static void __init preallocate_pgd_pages_range(unsigned long start, unsigned lon
 	panic("Failed to pre-allocate %s pages for %s area\n", lvl, area);
 }
 
-#define PAGE_END KASAN_SHADOW_START
+#define PAGE_END (PAGE_OFFSET + KERN_VIRT_SIZE)
 
 void __init pgtable_cache_init(void)
 {
-- 
2.45.1