Re: [PATCH v2 1/2] xen/riscv: add EMBEDDED_EXTRA_CFLAGS to CFLAGS

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.03.2023 14:22, Oleksii Kurochko wrote:
> The patch is needed to keep all address of cpu0_boot_stack
> PC-relative.
> 
> Pseudoinstruction 'la' can be transformed to 'auipc/addi' or
> 'auipc/l{w|d}'. It depends on the .option directive: nopic and pic
> or compiler flags.
> 
> Right now, 'la' transforms to 'auipc/l{w|d}', which in case of
> cpu0_boot_stack[] will lead to the usage of _GLOBAL_OFFSET_TABLE_
> where all addresses will be without counting that it might happen
> that linker address != load address ( so addresses inside got
> sections will be relative to linker time ).
> 
> It happens becuase the compiler from riscv64 docker compiled with
> --enable-default-pie:
>   [user@49295ae49cbe build]$ riscv64-linux-gnu-gcc -v
>   Using built-in specs.
>   COLLECT_GCC=riscv64-linux-gnu-gcc
>   COLLECT_LTO_WRAPPER=/usr/lib/gcc/riscv64-linux-gnu/12.2.0/lto-wrapper
>   Target: riscv64-linux-gnu
>   Configured with: /build/riscv64-linux-gnu-gcc/src/gcc-12.2.0/configure
>   --prefix=/usr --program-prefix=riscv64-linux-gnu- --with-local-
>   prefix=/usr/riscv64-linux-gnu --with-sysroot=/usr/riscv64-linux-gnu --
>   with-build-sysroot=/usr/riscv64-linux-gnu --libdir=/usr/lib --
>   libexecdir=/usr/lib --target=riscv64-linux-gnu --host=x86_64-pc-linux-
>   gnu --build=x86_64-pc-linux-gnu --with-system-zlib --with-isl --with-
>   linker-hash-style=gnu --disable-nls --disable-libunwind-exceptions --
>   disable-libstdcxx-pch --disable-libssp --disable-multilib --disable-
>   werror --enable-languages=c,c++ --enable-shared --enable-threads=posix
>   --enable-__cxa_atexit --enable-clocale=gnu --enable-gnu-unique-object -
>   -enable-linker-build-id --enable-lto --enable-plugin --enable-install-
>   libiberty --enable-gnu-indirect-function --enable-default-pie --enable-
>   checking=release
>   Thread model: posix
>   Supported LTO compression algorithms: zlib zstd
>   gcc version 12.2.0 (GCC)
> 
> Looking at gcc spec file for the RISC-V architecture:
>   [user@49295ae49cbe build]$ riscv64-linux-gnu-gcc -dumpspecs | grep -i
>   pic
>   --traditional-format %(subtarget_asm_debugging_spec) %{fno-pie|fno-
>   PIE|fno-pic|fno-PIC:;:-fpic} %{march=*} %{mabi=*} %{mno-relax} %{mbig-
>   endian} %{mlittle-endian} %(subtarget_asm_spec)%{misa-spec=*}
> which means that -fpic is enabled if none of the following options are
> present on the command line:
>     -fno-pie
>     -fno-PIE
>     -fno-pic
>     -fno-PIC
> 
> That's the reasons why 'la' is transformed to 'aupic/l{w|d} GOT' and
> not be dependent on the toolchain used.
> 
> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

There may also want to be Suggested-by to both Andrew and me.

Jan