Messages in this thread | | | Date | Thu, 18 Aug 2022 09:45:39 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86/build: Move '-mindirect-branch-cs-prefix' out of GCC-only block |
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On Wed, Aug 17, 2022 at 03:08:01PM -0700, Nick Desaulniers wrote: > On Wed, Aug 17, 2022 at 11:56 AM Nathan Chancellor <nathan@kernel.org> wrote: > > > > LLVM 16 will have support for this flag so move it out of the GCC-only > > block to allow LLVM builds to take advantage of it. > > > > Link: https://github.com/ClangBuiltLinux/linux/issues/1665 > > Link: https://github.com/llvm/llvm-project/commit/6f867f9102838ebe314c1f3661fdf95700386e5a > > Signed-off-by: Nathan Chancellor <nathan@kernel.org> > > Thanks for the patch, I also boot tested this (in virtual guests for > both 64b and 32b). > > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> > Tested-by: Nick Desaulniers <ndesaulniers@google.com> > > Peter, is there anything else special about these prefixes needed to > make use of them?
The thing to do is boot with (warning, *very* verbose):
"spectre_v2=retpoline,lfence debug-alternative"
and observe that the retpoline sites all replace:
"cs call __x86_indirect_thunk_r11" (6 bytes)
with:
"lfence; jmp *%r11" (6 bytes)
This being clang, you'll ofcourse still have a few weird:
"Jcc.d32 __x86_indirect_thunk_r11"
sites that will not be able to be replaced, but there's nothing I can do about that other than to continue to encourage y'all to stop emitting them ;-)
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