On 1/4/2018 2:05 PM, David Woodhouse wrote: > On Thu, 2018-01-04 at 14:00 -0600, Tom Lendacky wrote: >> Yes, lfence is sufficient. As long as the target is in the register >> before the lfence and we jump through the register all is good, i.e.: > > Thanks. Can I have a Reviewed-by: for this then please:
Reviewed-by: Tom Lendacky <thomas.lenda...@amd.com> While this works, a more efficient way to do the lfence support would be to not use the retpoline in this case. Changing the indirect jumps to do the "mov [rax], rax; lfence; jmp *rax" sequence would be quicker. I'm not sure if this is feasible given the need to do a retpoline if you can't use lfence, though. Thanks, Tom > > http://git.infradead.org/users/dwmw2/linux-retpoline.git/commitdiff/08d9eda03 > > From: David Woodhouse <d...@amazon.co.uk> > Date: Thu, 4 Jan 2018 20:01:53 +0000 > Subject: [PATCH] x86/retpoline: Simplify AMD variant of retpoline thunk > > On AMD (which is X86_FEATURE_K8), just the lfence is sufficient. > > Signed-off-by: David Woodhouse <d...@amazon.co.uk> > --- > arch/x86/lib/retpoline.S | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S > index bbdda5cc136e..26070976bff0 100644 > --- a/arch/x86/lib/retpoline.S > +++ b/arch/x86/lib/retpoline.S > @@ -11,7 +11,7 @@ > > 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_K8, __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE > 1: > lfence > jmp 1b > -- > 2.14.3 >