tree:   https://github.com/intel/tdx.git guest-filter
head:   0a555e67b143701a81612d819e693cf5786de418
commit: f0270b8da57a2d69e1eb8aa9c79ad9b6a01c90ab [10/28] x86/tdx: Add command line option to disable TDX guest filter support
config: x86_64-defconfig
compiler: gcc-11 (Debian 11.3.0-3) 11.3.0
reproduce (this is a W=1 build):
        # https://github.com/intel/tdx/commit/f0270b8da57a2d69e1eb8aa9c79ad9b6a01c90ab
        git remote add intel-tdx https://github.com/intel/tdx.git
        git fetch --no-tags intel-tdx guest-filter
        git checkout f0270b8da57a2d69e1eb8aa9c79ad9b6a01c90ab
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash

If you fix the issue, kindly add following tag where applicable
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   arch/x86/kernel/head64.c: In function 'x86_64_start_kernel':
>> arch/x86/kernel/head64.c:527:9: error: implicit declaration of function 'tdx_filter_init' [-Werror=implicit-function-declaration]
     527 |         tdx_filter_init();
         |         ^~~~~~~~~~~~~~~
   cc1: some warnings being treated as errors


vim +/tdx_filter_init +527 arch/x86/kernel/head64.c

   471	
   472	asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
   473	{
   474		/*
   475		 * Build-time sanity checks on the kernel image and module
   476		 * area mappings. (these are purely build-time and produce no code)
   477		 */
   478		BUILD_BUG_ON(MODULES_VADDR < __START_KERNEL_map);
   479		BUILD_BUG_ON(MODULES_VADDR - __START_KERNEL_map < KERNEL_IMAGE_SIZE);
   480		BUILD_BUG_ON(MODULES_LEN + KERNEL_IMAGE_SIZE > 2*PUD_SIZE);
   481		BUILD_BUG_ON((__START_KERNEL_map & ~PMD_MASK) != 0);
   482		BUILD_BUG_ON((MODULES_VADDR & ~PMD_MASK) != 0);
   483		BUILD_BUG_ON(!(MODULES_VADDR > __START_KERNEL));
   484		MAYBE_BUILD_BUG_ON(!(((MODULES_END - 1) & PGDIR_MASK) ==
   485					(__START_KERNEL & PGDIR_MASK)));
   486		BUILD_BUG_ON(__fix_to_virt(__end_of_fixed_addresses) <= MODULES_END);
   487	
   488		cr4_init_shadow();
   489	
   490		/* Kill off the identity-map trampoline */
   491		reset_early_page_tables();
   492	
   493		clear_bss();
   494	
   495		/*
   496		 * This needs to happen *before* kasan_early_init() because latter maps stuff
   497		 * into that page.
   498		 */
   499		clear_page(init_top_pgt);
   500	
   501		/*
   502		 * SME support may update early_pmd_flags to include the memory
   503		 * encryption mask, so it needs to be called before anything
   504		 * that may generate a page fault.
   505		 */
   506		sme_early_init();
   507	
   508		kasan_early_init();
   509	
   510		/*
   511		 * Flush global TLB entries which could be left over from the trampoline page
   512		 * table.
   513		 *
   514		 * This needs to happen *after* kasan_early_init() as KASAN-enabled .configs
   515		 * instrument native_write_cr4() so KASAN must be initialized for that
   516		 * instrumentation to work.
   517		 */
   518		__native_tlb_flush_global(this_cpu_read(cpu_tlbstate.cr4));
   519	
   520		idt_setup_early_handler();
   521	
   522		/* Needed before cc_platform_has() can be used for TDX */
   523		tdx_early_init();
   524	
   525		copy_bootdata(__va(real_mode_data));
   526	
 > 527		tdx_filter_init();
   528	
   529		/*
   530		 * Load microcode early on BSP.
   531		 */
   532		load_ucode_bsp();
   533	
   534		/* set init_top_pgt kernel high mapping*/
   535		init_top_pgt[511] = early_top_pgt[511];
   536	
   537		x86_64_start_reservations(real_mode_data);
   538	}
   539	

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