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Date:   Thu, 20 Apr 2023 10:36:43 -0700 (PDT)
Subject:     Re: [PATCH v9 1/1] riscv: Allow to downgrade paging mode from the command line
In-Reply-To: <20230329050951.66085-2-alexghiti@rivosinc.com>
CC:     corbet@lwn.net, Paul Walmsley <paul.walmsley@sifive.com>,
        aou@eecs.berkeley.edu, Conor Dooley <conor@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>, bjorn@kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-riscv@lists.infradead.org, alexghiti@rivosinc.com,
        Bjorn Topel <bjorn@rivosinc.com>
From:   Palmer Dabbelt <palmer@dabbelt.com>
To:     alexghiti@rivosinc.com
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On Tue, 28 Mar 2023 22:09:51 PDT (-0700), alexghiti@rivosinc.com wrote:
> 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 aacd149b6238 ("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, pie).
>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> Tested-by: Björn Töpel <bjorn@rivosinc.com>
> Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  5 +-
>  arch/riscv/kernel/Makefile                    |  2 +
>  arch/riscv/kernel/pi/Makefile                 | 39 ++++++++++++
>  arch/riscv/kernel/pi/cmdline_early.c          | 62 +++++++++++++++++++
>  arch/riscv/kernel/vmlinux.lds.S               |  8 +++
>  arch/riscv/lib/memcpy.S                       |  2 +
>  arch/riscv/lib/memmove.S                      |  2 +
>  arch/riscv/lib/strlen.S                       |  1 +
>  arch/riscv/mm/init.c                          | 36 +++++++++--
>  9 files changed, 150 insertions(+), 7 deletions(-)
>  create mode 100644 arch/riscv/kernel/pi/Makefile
>  create mode 100644 arch/riscv/kernel/pi/cmdline_early.c
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 6221a1d057dd..accc400b43f1 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/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.
> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> index 4cf303a779ab..aa22f87faeae 100644
> --- a/arch/riscv/kernel/Makefile
> +++ b/arch/riscv/kernel/Makefile
> @@ -89,3 +89,5 @@ 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/
> diff --git a/arch/riscv/kernel/pi/Makefile b/arch/riscv/kernel/pi/Makefile
> new file mode 100644
> index 000000000000..42c58f4ab53b
> --- /dev/null
> +++ b/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)) \
> +		   -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))
> diff --git a/arch/riscv/kernel/pi/cmdline_early.c b/arch/riscv/kernel/pi/cmdline_early.c
> new file mode 100644
> index 000000000000..05652d13c746
> --- /dev/null
> +++ b/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);
> +}
> diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
> index 615ff5842690..b12a843ad426 100644
> --- a/arch/riscv/kernel/vmlinux.lds.S
> +++ b/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 */
>  	}
> diff --git a/arch/riscv/lib/memcpy.S b/arch/riscv/lib/memcpy.S
> index 51ab716253fa..1a40d01a9543 100644
> --- a/arch/riscv/lib/memcpy.S
> +++ b/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)
> diff --git a/arch/riscv/lib/memmove.S b/arch/riscv/lib/memmove.S
> index e0609e1f0864..838ff2022fe3 100644
> --- a/arch/riscv/lib/memmove.S
> +++ b/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)
> diff --git a/arch/riscv/lib/strlen.S b/arch/riscv/lib/strlen.S
> index 15bb8f3aa959..9d0055616f7b 100644
> --- a/arch/riscv/lib/strlen.S
> +++ b/arch/riscv/lib/strlen.S
> @@ -131,3 +131,4 @@ strlen_zbb:
>  .option pop
>  #endif
>  SYM_FUNC_END(strlen)
> +SYM_FUNC_ALIAS(__pi_strlen, strlen)
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index bce899b180cd..3ad771571c2d 100644
> --- a/arch/riscv/mm/init.c
> +++ b/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
>
>  	/*

Alex found that we're missing -fpie, which was causing the LLVM boot 
failures.  I'm just going to squash in

diff --git a/arch/riscv/kernel/pi/Makefile b/arch/riscv/kernel/pi/Makefile
index 42c58f4ab53b..5d7cb991f2b8 100644
--- a/arch/riscv/kernel/pi/Makefile
+++ b/arch/riscv/kernel/pi/Makefile
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 # This file was copied from arm64/kernel/pi/Makefile.
 
-KBUILD_CFLAGS	:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
+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 \