riscv: Allow to downgrade paging mode from the command line

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.

In KASAN kernels, we can't use the libfdt that early in the boot process
since we are not ready to execute instrumented functions. So instead of
using the "generic" libfdt, we compile our own versions of those functions
that are not instrumented and that are prefixed so that they do not
conflict with the generic ones. We also need the non-instrumented versions
of the string functions and the prefixed versions of memcpy/memmove.

This is largely inspired by commit aacd149b62 ("arm64: head: avoid
relocating the kernel twice for KASLR") from which I removed compilation
flags that were not relevant to RISC-V at the moment (LTO, SCS). Also
note that we have to link with -z norelro to avoid ld.lld to throw a
warning with the new .got sections, like in commit 311bea3cb9 ("arm64:
link with -z norelro for LLD or aarch64-elf").

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20230424092313.178699-2-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -3576,7 +3576,10 @@
 			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.
+
+	no5lvl		[X86-64,RISCV] Disable 5-level paging mode. Forces
 			kernel to use 4-level paging instead.
 
 	nofsgsbase	[X86] Disables FSGSBASE instructions.

arch/riscv/Makefile

@@ -7,8 +7,9 @@
 #
 
 OBJCOPYFLAGS    := -O binary
+LDFLAGS_vmlinux := -z norelro
 ifeq ($(CONFIG_RELOCATABLE),y)
-	LDFLAGS_vmlinux += -shared -Bsymbolic -z notext -z norelro --emit-relocs
+	LDFLAGS_vmlinux += -shared -Bsymbolic -z notext --emit-relocs
 	KBUILD_CFLAGS += -fPIE
 endif
 ifeq ($(CONFIG_DYNAMIC_FTRACE),y)

arch/riscv/kernel/Makefile

@@ -87,3 +87,5 @@ obj-$(CONFIG_EFI)		+= efi.o
 obj-$(CONFIG_COMPAT)		+= compat_syscall_table.o
 obj-$(CONFIG_COMPAT)		+= compat_signal.o
 obj-$(CONFIG_COMPAT)		+= compat_vdso/
+
+obj-$(CONFIG_64BIT)		+= pi/

arch/riscv/kernel/pi/Makefile

@@ -0,0 +1,39 @@
+# SPDX-License-Identifier: GPL-2.0
+# This file was copied from arm64/kernel/pi/Makefile.
+
+KBUILD_CFLAGS	:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) -fpie \
+		   -Os -DDISABLE_BRANCH_PROFILING $(DISABLE_STACKLEAK_PLUGIN) \
+		   $(call cc-option,-mbranch-protection=none) \
+		   -I$(srctree)/scripts/dtc/libfdt -fno-stack-protector \
+		   -D__DISABLE_EXPORTS -ffreestanding \
+		   -fno-asynchronous-unwind-tables -fno-unwind-tables \
+		   $(call cc-option,-fno-addrsig)
+
+KBUILD_CFLAGS	+= -mcmodel=medany
+
+CFLAGS_cmdline_early.o += -D__NO_FORTIFY
+CFLAGS_lib-fdt_ro.o += -D__NO_FORTIFY
+
+GCOV_PROFILE	:= n
+KASAN_SANITIZE	:= n
+KCSAN_SANITIZE	:= n
+UBSAN_SANITIZE	:= n
+KCOV_INSTRUMENT	:= n
+
+$(obj)/%.pi.o: OBJCOPYFLAGS := --prefix-symbols=__pi_ \
+			       --remove-section=.note.gnu.property \
+			       --prefix-alloc-sections=.init
+$(obj)/%.pi.o: $(obj)/%.o FORCE
+	$(call if_changed,objcopy)
+
+$(obj)/lib-%.o: $(srctree)/lib/%.c FORCE
+	$(call if_changed_rule,cc_o_c)
+
+$(obj)/string.o: $(srctree)/lib/string.c FORCE
+	$(call if_changed_rule,cc_o_c)
+
+$(obj)/ctype.o: $(srctree)/lib/ctype.c FORCE
+	$(call if_changed_rule,cc_o_c)
+
+obj-y		:= cmdline_early.pi.o string.pi.o ctype.pi.o lib-fdt.pi.o lib-fdt_ro.pi.o
+extra-y		:= $(patsubst %.pi.o,%.o,$(obj-y))

arch/riscv/kernel/pi/cmdline_early.c

@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/types.h>
+#include <linux/init.h>
+#include <linux/libfdt.h>
+#include <linux/string.h>
+#include <asm/pgtable.h>
+#include <asm/setup.h>
+
+static char early_cmdline[COMMAND_LINE_SIZE];
+
+/*
+ * Declare the functions that are exported (but prefixed) here so that LLVM
+ * does not complain it lacks the 'static' keyword (which, if added, makes
+ * LLVM complain because the function is actually unused in this file).
+ */
+u64 set_satp_mode_from_cmdline(uintptr_t dtb_pa);
+
+static char *get_early_cmdline(uintptr_t dtb_pa)
+{
+	const char *fdt_cmdline = NULL;
+	unsigned int fdt_cmdline_size = 0;
+	int chosen_node;
+
+	if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
+		chosen_node = fdt_path_offset((void *)dtb_pa, "/chosen");
+		if (chosen_node >= 0) {
+			fdt_cmdline = fdt_getprop((void *)dtb_pa, chosen_node,
+						  "bootargs", NULL);
+			if (fdt_cmdline) {
+				fdt_cmdline_size = strlen(fdt_cmdline);
+				strscpy(early_cmdline, fdt_cmdline,
+					COMMAND_LINE_SIZE);
+			}
+		}
+	}
+
+	if (IS_ENABLED(CONFIG_CMDLINE_EXTEND) ||
+	    IS_ENABLED(CONFIG_CMDLINE_FORCE) ||
+	    fdt_cmdline_size == 0 /* CONFIG_CMDLINE_FALLBACK */) {
+		strncat(early_cmdline, CONFIG_CMDLINE,
+			COMMAND_LINE_SIZE - fdt_cmdline_size);
+	}
+
+	return early_cmdline;
+}
+
+static u64 match_noXlvl(char *cmdline)
+{
+	if (strstr(cmdline, "no4lvl"))
+		return SATP_MODE_48;
+	else if (strstr(cmdline, "no5lvl"))
+		return SATP_MODE_57;
+
+	return 0;
+}
+
+u64 set_satp_mode_from_cmdline(uintptr_t dtb_pa)
+{
+	char *cmdline = get_early_cmdline(dtb_pa);
+
+	return match_noXlvl(cmdline);
+}

arch/riscv/kernel/vmlinux.lds.S

@@ -83,6 +83,14 @@ SECTIONS
 	/* Start of init data section */
 	__init_data_begin = .;
 	INIT_DATA_SECTION(16)
+
+	/* Those sections result from the compilation of kernel/pi/string.c */
+	.init.pidata : {
+		*(.init.srodata.cst8*)
+		*(.init__bug_table*)
+		*(.init.sdata*)
+	}
+
 	.init.bss : {
 		*(.init.bss)	/* from the EFI stub */
 	}
@@ -128,9 +136,10 @@ SECTIONS
 		__rela_dyn_end = .;
 	}
 
+	.got : { *(.got*) }
+
 #ifdef CONFIG_RELOCATABLE
 	.data.rel : { *(.data.rel*) }
-	.got : { *(.got*) }
 	.plt : { *(.plt) }
 	.dynamic : { *(.dynamic) }
 	.dynsym : { *(.dynsym) }

arch/riscv/lib/memcpy.S

@@ -106,3 +106,5 @@ WEAK(memcpy)
 6:
 	ret
 END(__memcpy)
+SYM_FUNC_ALIAS(__pi_memcpy, __memcpy)
+SYM_FUNC_ALIAS(__pi___memcpy, __memcpy)

arch/riscv/lib/memmove.S

@@ -314,3 +314,5 @@ return_from_memmove:
 
 SYM_FUNC_END(memmove)
 SYM_FUNC_END(__memmove)
+SYM_FUNC_ALIAS(__pi_memmove, __memmove)
+SYM_FUNC_ALIAS(__pi___memmove, __memmove)

arch/riscv/lib/strlen.S

@@ -130,3 +130,4 @@ strlen_zbb:
 .option pop
 #endif
 SYM_FUNC_END(strlen)
+SYM_FUNC_ALIAS(__pi_strlen, strlen)

arch/riscv/mm/init.c

@@ -746,6 +746,8 @@ static __init pgprot_t pgprot_from_va(uintptr_t va)
 #endif /* CONFIG_STRICT_KERNEL_RWX */
 
 #if defined(CONFIG_64BIT) && !defined(CONFIG_XIP_KERNEL)
+u64 __pi_set_satp_mode_from_cmdline(uintptr_t dtb_pa);
+
 static void __init disable_pgtable_l5(void)
 {
 	pgtable_l5_enabled = false;
@@ -760,17 +762,39 @@ static void __init disable_pgtable_l4(void)
 	satp_mode = SATP_MODE_39;
 }
 
+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);
+
 /*
  * 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;
+	u64 satp_mode_cmdline = __pi_set_satp_mode_from_cmdline(dtb_pa);
+
+	if (satp_mode_cmdline == SATP_MODE_57) {
+		disable_pgtable_l5();
+	} else if (satp_mode_cmdline == SATP_MODE_48) {
+		disable_pgtable_l5();
+		disable_pgtable_l4();
+		return;
+	}
 
 	create_p4d_mapping(early_p4d,
 			set_satp_mode_pmd, (uintptr_t)early_pud,
@@ -789,7 +813,8 @@ 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;
@@ -800,9 +825,8 @@ 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;
 		}
@@ -1031,7 +1055,7 @@ 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
 
 	/*