Re: [PATCH v4 1/1] riscv: Allow to downgrade paging mode from the command line

["Björn Töpel" <bjorn@kernel.org>]

Alexandre Ghiti <alexghiti@rivosinc.com> writes:

> 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>
> ---
>  .../admin-guide/kernel-parameters.txt         |  5 +-
>  arch/riscv/kernel/Makefile                    |  2 +
>  arch/riscv/kernel/pi/Makefile                 | 34 ++++++++++++
>  arch/riscv/kernel/pi/cmdline_early.c          | 52 +++++++++++++++++++
>  arch/riscv/lib/memcpy.S                       |  2 +
>  arch/riscv/lib/memmove.S                      |  2 +
>  arch/riscv/mm/init.c                          | 36 ++++++++++---
>  7 files changed, 126 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 6cfa6e3996cf..1d8fc66d2b0d 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3578,7 +3578,10 @@
>  			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..6756209f1ac6 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-y				+= pi/
> diff --git a/arch/riscv/kernel/pi/Makefile b/arch/riscv/kernel/pi/Makefile
> new file mode 100644
> index 000000000000..554e936ef0b6
> --- /dev/null
> +++ b/arch/riscv/kernel/pi/Makefile
> @@ -0,0 +1,34 @@
> +# 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 \
> +		   $(call cc-option,-fno-addrsig)
> +
> +GCOV_PROFILE	:= n
> +KASAN_SANITIZE	:= n
> +KCSAN_SANITIZE	:= n
> +UBSAN_SANITIZE	:= n
> +KCOV_INSTRUMENT	:= n
> +
> +$(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..5ae7b853fa66
> --- /dev/null
> +++ b/arch/riscv/kernel/pi/cmdline_early.c
> @@ -0,0 +1,52 @@
> +// 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] __initdata;
> +
> +static char * __init 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);

Alex and I have been running some more tests, and fdt_cmdline is missing
a NULL-check here.

We'll need a v5 to fix this!


Björn