[RFC PATCH V1 02/11] riscv: Introduce concept of hardware base page

[Xu Lu]

The key idea to implement larger base page based on MMU that only
supports 4K page is to decouple the MMU page from the software page in
view of kernel mm. In contrary to software page, we denote the MMU page
as hardware page.

To decouple these two kinds of pages, we should manage, allocate and map
memory at a granularity of software page, which is exactly what existing
mm code does. The page table operations, however, should configure page
table entries at a granularity of hardware page, which is the
responsibility of arch code.

This commit introduces the concept of hardware base page for RISCV.

Signed-off-by: Xu Lu <luxu.kernel at bytedance.com>
---
 arch/riscv/Kconfig            | 8 ++++++++
 arch/riscv/include/asm/page.h | 6 +++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 95a2a06acc6a..105cbb3ca797 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -221,6 +221,14 @@ config PAGE_OFFSET
 	default 0x80000000 if !MMU
 	default 0xff60000000000000 if 64BIT
 
+config RISCV_HW_PAGE_SHIFT
+	int
+	default 12
+
+config RISCV_PAGE_SHIFT
+	int
+	default 12
+
 config KASAN_SHADOW_OFFSET
 	hex
 	depends on KASAN_GENERIC
diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
index 57e887bfa34c..a8c59d80683c 100644
--- a/arch/riscv/include/asm/page.h
+++ b/arch/riscv/include/asm/page.h
@@ -12,7 +12,11 @@
 #include <linux/pfn.h>
 #include <linux/const.h>
 
-#define PAGE_SHIFT	(12)
+#define HW_PAGE_SHIFT	CONFIG_RISCV_HW_PAGE_SHIFT
+#define HW_PAGE_SIZE	(_AC(1, UL) << HW_PAGE_SHIFT)
+#define HW_PAGE_MASK	(~(HW_PAGE_SIZE - 1))
+
+#define PAGE_SHIFT	CONFIG_RISCV_PAGE_SHIFT
 #define PAGE_SIZE	(_AC(1, UL) << PAGE_SHIFT)
 #define PAGE_MASK	(~(PAGE_SIZE - 1))
 
-- 
2.20.1