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objtool complains: arch/x86/kvm/kvm.o: warning: objtool: .altinstr_replacement+0xc5: call without frame pointer save/setup vmlinux.o: warning: objtool: .altinstr_replacement+0x2eb: call without frame pointer save/setup Make sure %rSP is an output operand to the respective asm() statements. The test_cc() hunk and ALT_OUTPUT_SP() courtesy of peterz. Also from him add some helpful debugging info to the documentation. Now on to the explanations: tl;dr: The alternatives macros are pretty fragile. If I do ALT_OUTPUT_SP(output) in order to be able to package in a %rsp reference for objtool so that a stack frame gets properly generated, the inline asm input operand with positional argument 0 in clear_page(): "0" (page) gets "renumbered" due to the added : "+r" (current_stack_pointer), "=D" (page) and then gcc says: ./arch/x86/include/asm/page_64.h:53:9: error: inconsistent operand constraints in an ‘asm’ The fix is to use an explicit "D" constraint which points to a singleton register class (gcc terminology) which ends up doing what is expected here: the page pointer - input and output - should be in the same %rdi register. Other register classes have more than one register in them - example: "r" and "=r" or "A": ‘A’ The ‘a’ and ‘d’ registers. This class is used for instructions that return double word results in the ‘ax:dx’ register pair. Single word values will be allocated either in ‘ax’ or ‘dx’. so using "D" and "=D" just works in this particular case. And yes, one would say, sure, why don't you do "+D" but then: : "+r" (current_stack_pointer), "+D" (page) : [old] "i" (clear_page_orig), [new1] "i" (clear_page_rep), [new2] "i" (clear_page_erms), : "cc", "memory", "rax", "rcx") now find the Waldo^Wcomma which throws a wrench into all this. Because that silly macro has an "input..." consume-all last macro arg and in it, one is supposed to supply input *and* clobbers, leading to silly syntax snafus. Yap, they need to be cleaned up, one fine day... Closes: https://lore.kernel.org/oe-kbuild-all/202406141648.jO9qNGLa-lkp@intel.com/ Reported-by: kernel test robot <lkp@intel.com> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de> Acked-by: Sean Christopherson <seanjc@google.com> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lore.kernel.org/r/20240625112056.GDZnqoGDXgYuWBDUwu@fat_crate.local
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.