riscv: Improve kasan definitions

There is no functional change here, only improvement in code readability
by adding comments to explain where the kasan constants come from and by
replacing hardcoded numerical constant by the corresponding define.

Note that the comments come from arm64.
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/kasan.h

@@ -8,12 +8,28 @@
 
 #ifdef CONFIG_KASAN
 
+/*
+ * The following comment was copied from arm64:
+ * KASAN_SHADOW_START: beginning of the kernel virtual addresses.
+ * KASAN_SHADOW_END: KASAN_SHADOW_START + 1/N of kernel virtual addresses,
+ * where N = (1 << KASAN_SHADOW_SCALE_SHIFT).
+ *
+ * KASAN_SHADOW_OFFSET:
+ * This value is used to map an address to the corresponding shadow
+ * address by the following formula:
+ *     shadow_addr = (address >> KASAN_SHADOW_SCALE_SHIFT) + KASAN_SHADOW_OFFSET
+ *
+ * (1 << (64 - KASAN_SHADOW_SCALE_SHIFT)) shadow addresses that lie in range
+ * [KASAN_SHADOW_OFFSET, KASAN_SHADOW_END) cover all 64-bits of virtual
+ * addresses. So KASAN_SHADOW_OFFSET should satisfy the following equation:
+ *      KASAN_SHADOW_OFFSET = KASAN_SHADOW_END -
+ *                              (1ULL << (64 - KASAN_SHADOW_SCALE_SHIFT))
+ */
 #define KASAN_SHADOW_SCALE_SHIFT	3
 
-#define KASAN_SHADOW_SIZE	(UL(1) << (38 - KASAN_SHADOW_SCALE_SHIFT))
-#define KASAN_SHADOW_START	KERN_VIRT_START /* 2^64 - 2^38 */
+#define KASAN_SHADOW_SIZE	(UL(1) << ((CONFIG_VA_BITS - 1) - KASAN_SHADOW_SCALE_SHIFT))
+#define KASAN_SHADOW_START	KERN_VIRT_START
 #define KASAN_SHADOW_END	(KASAN_SHADOW_START + KASAN_SHADOW_SIZE)
-
 #define KASAN_SHADOW_OFFSET	(KASAN_SHADOW_END - (1ULL << \
 					(64 - KASAN_SHADOW_SCALE_SHIFT)))