* Dan Williams <dan.j.willi...@intel.com> wrote: > + /* > + * Sanitize extra registers of values that a speculation attack > + * might want to exploit. In the CONFIG_FRAME_POINTER=y case, > + * the expectation is that %ebp will be clobbered before it > + * could be used. > + */ > + .macro CLEAR_EXTRA_REGS_NOSPEC > + xorq %r15, %r15 > + xorq %r14, %r14 > + xorq %r13, %r13 > + xorq %r12, %r12 > + xorl %ebx, %ebx > +#ifndef CONFIG_FRAME_POINTER > + xorl %ebp, %ebp > +#endif
BTW., is there any reason behind the order of the clearing of these registers? This ordering seems rather random: - The canonical register order is: RBX, RBP, R12, R13, R14, R15, which is also their push-order on the stack. - The CLEAR_EXTRA_REGS_NOSPEC order appears to be the reverse order (pop-order), but with RBX and RBP reversed. So since this is a 'push side' primitive I'd use the regular (push-) ordering instead: .macro CLEAR_EXTRA_REGS_NOSPEC xorl %ebx, %ebx xorl %ebp, %ebp xorq %r12, %r12 xorq %r13, %r13 xorq %r14, %r14 xorq %r15, %r15 It obviously doesn't matter to correctness - only to readability. There's also a (very) small micro-optimization argument in favor of the regular order: the earlier registers are more likely to be utilized by C functions, so the sooner we clear them, the less potential interaction these clearing instructions are going to have with any later use. Thanks, Ingo