[PATCH v3 13/13] riscv: Allow user to downgrade to sv39 when hw supports sv48 if !KASAN

[Alexandre Ghiti]

This is made possible by using the mmu-type property of the cpu node of
the device tree.

By default, the kernel will boot with 4-level page table if the hw supports
it but it can be interesting for the user to select 3-level page table as
it is less memory consuming and faster since it requires less memory
accesses in case of a TLB miss.

This functionality requires that kasan is disabled since calling the fdt
functions that are kasan instrumented with the MMU off can't work.

Signed-off-by: Alexandre Ghiti <alexandre.ghiti at canonical.com>
---
 arch/riscv/mm/init.c | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 28de6ea0a720..299b5a44f902 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -633,10 +633,38 @@ static void __init disable_pgtable_l4(void)
  * then read SATP to see if the configuration was taken into account
  * meaning sv48 is supported.
  */
-static __init void set_satp_mode(void)
+static __init void set_satp_mode(uintptr_t dtb_pa)
 {
 	u64 identity_satp, hw_satp;
 	uintptr_t set_satp_mode_pmd;
+#ifndef CONFIG_KASAN
+	/*
+	 * The below fdt functions are kasan instrumented, since at this point
+	 * there is no mapping for the kasan shadow memory, this can't be used
+	 * when kasan is enabled otherwise it traps.
+	 */
+	int cpus_node;
+
+	/* Check if the user asked for sv39 explicitly in the device tree */
+	cpus_node = fdt_path_offset((void *)dtb_pa, "/cpus");
+	if (cpus_node >= 0) {
+		int node;
+
+		fdt_for_each_subnode(node, (void *)dtb_pa, cpus_node) {
+			const char *mmu_type = fdt_getprop((void *)dtb_pa, node,
+					"mmu-type", NULL);
+			if (!mmu_type)
+				continue;
+
+			if (!strcmp(mmu_type, "riscv,sv39")) {
+				disable_pgtable_l4();
+				return;
+			}
+
+			break;
+		}
+	}
+#endif
 
 	set_satp_mode_pmd = ((unsigned long)set_satp_mode) & PMD_MASK;
 	create_pgd_mapping(early_pg_dir,
@@ -838,7 +866,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
 #endif
 
 #if defined(CONFIG_64BIT) && !defined(CONFIG_XIP_KERNEL)
-	set_satp_mode();
+	set_satp_mode(dtb_pa);
 #endif
 
 	kernel_map.va_pa_offset = PAGE_OFFSET - kernel_map.phys_addr;
-- 
2.32.0