[PATCH] riscv: mm: check the SV39 rule

[Cheng Chao]

SV39 rule: the address of bits[63..39] should be the same as bit[38],
it is easy to violate if configure PAGE_OFFSET too small.
for instance, PAGE_OFFSET=0xffffffc0_0000_0000,
the FIXADDR_START will be the 0xfffffffb0_xxxx_xxxx,
bit[39] == 0'b1, bit[38] == 0'b0, when access the FIXADDR,
which cause the page fault.
It's difficult to debug, check it when compile is necessary.

Signed-off-by: Cheng Chao <cs.os.kernel at gmail.com>
---
 arch/riscv/mm/init.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index bfa2dea95354..82d112c02662 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -1387,6 +1387,12 @@ static void __init arch_reserve_crashkernel(void)
 
 void __init paging_init(void)
 {
+#ifdef CONFIG_64BIT
+	BUILD_BUG_ON_MSG((VA_BITS == VA_BITS_SV39) &&
+			(((FIXADDR_START & BIT(39)) >> 39)
+			 != ((FIXADDR_START & BIT(38)) >> 38)),
+			"violate SV39 rule: bits[63..39] should be same as bit[38]");
+#endif
 	setup_bootmem();
 	setup_vm_final();
 
-- 
2.26.3