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address. >> + * @addr: address of kernel page >> + * >> + * Switch between three implementations of page clearing based on CPU >> + * capabilities: >> + * >> + * - __clear_pages_unrolled(): the oldest, slowest and universally >> + * supported method. Zeroes via 8-byte MOV instructions unrolled 8x >> + * to write a 64-byte cacheline in each loop iteration. >> + * >> + * - "REP; STOSQ": really old CPUs had crummy REP implementations. >> + * Vendor CPU setup code sets 'REP_GOOD' on CPUs where REP can be >> + * trusted. The instruction writes 8-byte per REP iteration but >> + * CPUs can internally batch these together and do larger writes. >> + * >> + * - "REP; STOSB": CPUs that enumerate 'ERMS' have an improved STOS >> + * implementation that is less picky about alignment and where >> + * STOSB (1-byte at a time) is actually faster than STOSQ (8-bytes >> + * at a time.) > > Please put here in BIG RED LETTERS something along the lines of: > > "The inline asm has a CALL instruction and usually that is a no-no due to the > compiler not knowing that there's a CALL inside the asm and thus won't track > callee-clobbered registers but in this case, all the callee clobbereds by > __clear_pages_unrolled() are part of the inline asm register specification so > that is fine. > > Just don't assume that you can call *any* function from inside asm due to the > above." Will add something clarifying this. >> + * >> + * Does absolutely no exception handling. >> + */ >> +static inline void clear_page(void *addr) >> { >> + u64 len = PAGE_SIZE; >> /* >> * Clean up KMSAN metadata for the page being cleared. The assembly call >> - * below clobbers @page, so we perform unpoisoning before it. >> + * below clobbers @addr, so we perform unpoisoning before it. > > s/we // > >> */ >> - kmsan_unpoison_memory(page, PAGE_SIZE); >> - alternative_call_2(clear_page_orig, >> - clear_page_rep, X86_FEATURE_REP_GOOD, >> - clear_page_erms, X86_FEATURE_ERMS, >> - "=D" (page), >> - "D" (page), >> - "cc", "memory", "rax", "rcx"); >> + kmsan_unpoison_memory(addr, len); >> + asm volatile(ALTERNATIVE_2("call __clear_pages_unrolled", >> + "shrq $3, %%rcx; rep stosq", X86_FEATURE_REP_GOOD, >> + "rep stosb", X86_FEATURE_ERMS) >> + : "+c" (len), "+D" (addr), ASM_CALL_CONSTRAINT >> + : "a" (0) >> + : "cc", "memory"); >> } > > With that: > > Reviewed-by: Borislav Petkov (AMD) Thanks! -- ankur