[PATCH resend v3] riscv: Allow to downgrade paging mode from the command line

[Alexandre Ghiti <alexghiti@rivosinc.com>]

Add 2 early command line parameters that allow to downgrade satp mode
(using the same naming as x86):
- "no5lvl": use a 4-level page table (down from sv57 to sv48)
- "no4lvl": use a 3-level page table (down from sv57/sv48 to sv39)

Note that going through the device tree to get the kernel command line
works with ACPI too since the efi stub creates a device tree anyway with
the command line.

Also, as those params are treated very early in the boot process and we
use standard device tree functions that may be kasan instrumented, we
only enable them for !KASAN configurations.

Reviewed-by: Björn Töpel <bjorn@kernel.org>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---

v3:
- Massage commit log to make no4lvl clearer, as asked by Conor
- Add a note to kernel-parameters.txt regarding the impossibility to use
  those parameters when KASAN is enabled, as suggested by Conor
- Add RB from Björn

v2:
- Honor CMDLINE_EXTEND and CMDLINE_FORCE as noticed by Björn

 .../admin-guide/kernel-parameters.txt         |  9 ++-
 arch/riscv/mm/init.c                          | 72 +++++++++++++++++--
 2 files changed, 74 insertions(+), 7 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6cfa6e3996cf..fd647412ec91 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3578,8 +3578,15 @@
 			emulation library even if a 387 maths coprocessor
 			is present.
 
-	no5lvl		[X86-64] Disable 5-level paging mode. Forces
+	no4lvl		[RISCV] Disable 4-level and 5-level paging modes. Forces
+			kernel to use 3-level paging instead.
+			Note: On RISC-V, this can't be used when KASAN is
+			enabled.
+
+	no5lvl		[X86-64,RISCV] Disable 5-level paging mode. Forces
 			kernel to use 4-level paging instead.
+			Note: On RISC-V, this can't be used when KASAN is
+			enabled.
 
 	nofsgsbase	[X86] Disables FSGSBASE instructions.
 
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 478d6763a01a..10f99fa74368 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -746,17 +746,77 @@ static void __init disable_pgtable_l4(void)
 	satp_mode = SATP_MODE_39;
 }
 
+#ifndef CONFIG_KASAN
+static __init bool match_noXlvl(const char *cmdline)
+{
+	if (strstr(cmdline, "no5lvl")) {
+		disable_pgtable_l5();
+	} else if (strstr(cmdline, "no4lvl")) {
+		disable_pgtable_l5();
+		disable_pgtable_l4();
+		return true;
+	}
+
+	return false;
+}
+
+static int __init print_no4lvl(char *p)
+{
+	pr_info("Disabled 4-level and 5-level paging");
+	return 0;
+}
+early_param("no4lvl", print_no4lvl);
+
+static int __init print_no5lvl(char *p)
+{
+	pr_info("Disabled 5-level paging");
+	return 0;
+}
+early_param("no5lvl", print_no5lvl);
+#endif
+
 /*
  * There is a simple way to determine if 4-level is supported by the
  * underlying hardware: establish 1:1 mapping in 4-level page table mode
  * 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 = ((unsigned long)set_satp_mode) & PMD_MASK;
-	bool check_l4 = false;
+
+#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.
+	 */
+	int chosen_node;
+	unsigned int fdt_cmdline_size = 0;
+
+	if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
+		chosen_node = fdt_path_offset((void *)dtb_pa, "/chosen");
+		if (chosen_node >= 0) {
+			const char *fdt_cmdline;
+
+			fdt_cmdline = fdt_getprop((void *)dtb_pa, chosen_node,
+						  "bootargs", NULL);
+			if (fdt_cmdline) {
+				if (match_noXlvl(fdt_cmdline))
+					return;
+				fdt_cmdline_size = strlen(fdt_cmdline);
+			}
+		}
+	}
+
+	if (IS_ENABLED(CONFIG_CMDLINE_EXTEND) ||
+	    IS_ENABLED(CONFIG_CMDLINE_FORCE) ||
+	    fdt_cmdline_size == 0 /* CONFIG_CMDLINE_FALLBACK */) {
+		if (match_noXlvl(CONFIG_CMDLINE))
+			return;
+	}
+#endif
 
 	create_p4d_mapping(early_p4d,
 			set_satp_mode_pmd, (uintptr_t)early_pud,
@@ -775,7 +835,8 @@ static __init void set_satp_mode(void)
 retry:
 	create_pgd_mapping(early_pg_dir,
 			   set_satp_mode_pmd,
-			   check_l4 ? (uintptr_t)early_pud : (uintptr_t)early_p4d,
+			   pgtable_l5_enabled ?
+				(uintptr_t)early_p4d : (uintptr_t)early_pud,
 			   PGDIR_SIZE, PAGE_TABLE);
 
 	identity_satp = PFN_DOWN((uintptr_t)&early_pg_dir) | satp_mode;
@@ -786,9 +847,8 @@ static __init void set_satp_mode(void)
 	local_flush_tlb_all();
 
 	if (hw_satp != identity_satp) {
-		if (!check_l4) {
+		if (pgtable_l5_enabled) {
 			disable_pgtable_l5();
-			check_l4 = true;
 			memset(early_pg_dir, 0, PAGE_SIZE);
 			goto retry;
 		}
@@ -979,7 +1039,7 @@ 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;
-- 
2.37.2