On 1/4/2018 5:47 PM, Tom Lendacky wrote: > 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 >
I do need to send the patches that make lfence a serializing instruction for AMD. I'll get those out as soon as I can. 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 >>