On Thu, Jan 04, 2018 at 02:36:58PM +0000, David Woodhouse wrote: > Enable the use of -mindirect-branch=thunk-extern in newer GCC, and provide > the corresponding thunks. Provide assembler macros for invoking the thunks > in the same way that GCC does, from native and inline assembler. > > This adds an X86_BUG_NO_RETPOLINE "feature" for runtime patching out > of the thunks. This is a placeholder for now; the patches which support > the new Intel/AMD microcode features will flesh out the precise conditions > under which we disable the retpoline and do other things instead. > > [Andi Kleen: Rename the macros and add CONFIG_RETPOLINE option] > > Signed-off-by: David Woodhouse <d...@amazon.co.uk> ... > +.macro THUNK sp reg > + .section .text.__x86.indirect_thunk.\reg > + > +ENTRY(__x86.indirect_thunk.\reg) > + CFI_STARTPROC > + ALTERNATIVE "call 2f", __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE > +1: > + lfence > + jmp 1b > +2: > + mov %\reg, (%\sp) > + ret > + CFI_ENDPROC > +ENDPROC(__x86.indirect_thunk.\reg)
Clearly Paul's approach to retpoline without lfence is faster. I'm guessing it wasn't shared with amazon/intel until now and this set of patches going to adopt it, right? Paul, could you share a link to a set of alternative gcc patches that do retpoline similar to llvm diff ?
