On Thu, 2018-01-04 at 15:29 +0000, Woodhouse, David wrote: > > > With the GCC -mindirect-branch=thunk-external support, and microcode, > > Xen will make a boot-time choice between using Retpoline, Lfence (which > > is the better AMD option, and more performant than retpoline), or IBRS > > on Skylake and newer processors where it is strictly necessary, as well > > as using IBPB whenever available. > > I need to pull in the AMD lfence alternative for retpoline, giving us a > 3-way choice of the existing retpoline thunk, "lfence; jmp *%\reg", and > a bare "jmp *%\reg".
I think I can abuse X86_FEATURE_SYSCALL for that, right? So it would look something like this: --- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -12,7 +12,7 @@ ENTRY(__x86.indirect_thunk.\reg) CFI_STARTPROC - ALTERNATIVE "call 2f", __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE + ALTERNATIVE_2 "call 2f", __stringify(lfence;jmp *%\reg), X86_FEATURE_SYSCALL, __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE 1: lfence ASM_UNREACHABLE However, I would very much like to see a categorical statement from AMD that the lfence is sufficient in all cases. Remember, Intel were saying that too for a while, before finding that it was not *quite* good enough.
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