Building libgcc for mcmodel=kernel: Difference between revisions

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Line 1: {{Rating\|2}} In [[X86-64\|x86_64]], if you link (e.g. through a linker script) your kernel directly to the higher half of the virtual memory, you might want to build the kernel with <tt>-mcmodel=kernel</tt> or <tt>-mcmodel=large</tt>. Also, if you link with <tt>libgcc</tt> and use <tt>crtbegin.o</tt> and <tt>crtend.o</tt> (see [[Calling_Global_Constructors\|Calling Global Constructors]]), you might wand to build <tt>libgcc</tt> with <tt>-mcmodel=kernel</tt> or <tt>-mcmodel-large</tt>. Otherwise, you may get errors when linking, either from <tt>crtstuff.c</tt> or from kernel code, like: <~~source~~syntaxhighlight lang="text"> .../crtbegin.o: In function `deregister_tm_clones': crtstuff.c:(.text+0x2): relocation truncated to fit: R_X86_64_32 against symbol `__TMC_END__' defined in .dtors section in ... Line 23 ⟶ 24: crtstuff.c:(.text+0x13): relocation truncated to fit: R_X86_64_32 against `.ctors' collect2: error: ld returned 1 exit status </syntaxhighlight> ~~</source>~~ One solution is to build a specific cross compiler, used only for kernel compilation (different from the one used for the userland), with <tt>libgcc</tt> (where <tt>crtbegin.o</tt> and <tt>crtend.o</tt> are generated, from <tt>crtstuff.c</tt>) compiled, like the kernel, with <tt>-mcmodel=kernel</tt> or <tt>-mcmodel=large</tt>. As far as we can tell, you could even mix the two modes, e.g.: compile the kernel with <tt>-mcmodel=kernel</tt> and <tt>libgcc</tt> with <tt>-mcmodel=large</tt> or the other way around. In this case we also use <tt>-mno-red-zone</tt> to compile <tt>libgcc</tt> because it may be needed for the kernel (see [[Libgcc_without_red_zone\|Libgcc without red zone]]). Afterwards, you can compile and link your kernel with the generated cross compiler using <tt>-mcmodel=kernel</tt> or <tt>-mcmodel=large</tt>, and <tt>-mno-red-zone</tt>, and link with <tt>crtbegin.o</tt> and <tt>crtend.o</tt>. These recipes were tested and work at least for GCC 6.3, 7 and ~~GCC 7.1~~8. Starting from the [[GCC_Cross-Compiler\|GCC Cross-Compiler]] recipe, and using <tt>TARGET=x86_64-elf</tt>, we can proceed as in the next sections. == Compiling libgcc with -mcmodel=kernel == Line 39 ⟶ 40: This may require a workaround. First compile <tt>binutils</tt> as usual, then add the binaries to the <tt>PATH</tt>, and start to compile <tt>gcc</tt>. When compiling <tt>libgcc</tt> with <tt>-mcmodel=kernel</tt>, it will fail. Then patch the <tt>Makefile</tt> to disable <tt>PIC</tt>, repeat and continue: <~~source~~syntaxhighlight lang='"bash'"> # (...) mkdir build Line 51 ⟶ 52: make install-gcc make install-target-libgcc </syntaxhighlight> ~~</source>~~ == Compiling libgcc with -mcmodel=large == Line 57 ⟶ 58: First compile <tt>binutils</tt> as usual, then add the binaries to the <tt>PATH</tt>, and start to compile <tt>gcc</tt>. Then compile <tt>libgcc</tt> with <tt>-mcmodel=large</tt>: <~~source~~syntaxhighlight lang='"bash'"> # (...) mkdir build Line 66 ⟶ 67: make install-gcc make install-target-libgcc </syntaxhighlight> ~~</source>~~ == See Also == === Articles === * [[GCC_Cross-Compiler\|GCC Cross-Compiler]] * [[Libgcc]] * [[~~Libgcc_without_red_zone~~Libgcc\|libgcc]] * [[Libgcc_without_red_zone\|libgcc without red zone]] * [[Calling_Global_Constructors\|Calling Global Constructors]] [[Category:Compilers]]