linux-kernel - Re: [PATCH v7 1/1] riscv: Allow to downgrade paging mode from the command line

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Date:   Mon, 27 Feb 2023 12:31:16 +0000
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Alexandre Ghiti <alexghiti@...osinc.com>
CC:     Jonathan Corbet <corbet@....net>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Conor Dooley <conor@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Björn Töpel <bjorn@...nel.org>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-riscv@...ts.infradead.org>,
        Björn Töpel <bjorn@...osinc.com>,
        <nathan@...nel.org>, <llvm@...ts.linux.dev>,
        <ndesaulniers@...gle.com>
Subject: Re: [PATCH v7 1/1] riscv: Allow to downgrade paging mode from the
 command line

Hey Alex, clang/llvm folk,

On Fri, Feb 24, 2023 at 11:02:18AM +0100, Alexandre Ghiti 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@...osinc.com>
> Tested-by: Björn Töpel <bjorn@...osinc.com>
> Reviewed-by: Björn Töpel <bjorn@...osinc.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  5 +-
>  arch/riscv/kernel/Makefile                    |  2 +
>  arch/riscv/kernel/pi/Makefile                 | 37 +++++++++++
>  arch/riscv/kernel/pi/cmdline_early.c          | 62 +++++++++++++++++++

+      4 ld.lld: warning: vmlinux.a(arch/riscv/kernel/pi/string.pi.o):(.init.sdata) is being placed in '.init.sdata'

I'm getting 4 new linker warnings with LLVM=1 clang-15 allmodconfig
builds - but I don't really understand this issue.
Is this spurious, and a just consequence of...

> diff --git a/arch/riscv/kernel/pi/Makefile b/arch/riscv/kernel/pi/Makefile
> new file mode 100644
> index 000000000000..4002ed94b6d3
> --- /dev/null
> +++ b/arch/riscv/kernel/pi/Makefile
> @@ -0,0 +1,37 @@
> +# 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 \
> +		   $(call cc-option,-fno-addrsig)
> +
> +CFLAGS_cmdline_early.o += -D__NO_FORTIFY
> +CFLAGS_lib-fdt_ro.o += -D__NO_FORTIFY
> +
> +GCOV_PROFILE	:= n
> +KASAN_SANITIZE	:= n
> +KCSAN_SANITIZE	:= n
> +UBSAN_SANITIZE	:= n
> +KCOV_INSTRUMENT	:= n
> +
> +$(obj)/%.pi.o: OBJCOPYFLAGS := --prefix-symbols=__pi_ \
> +			       --remove-section=.note.gnu.property \
> +			       --prefix-alloc-sections=.init

...this?

> +$(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))

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