Re: [PATCH] x86/speculation: Use AMD specific retpoline for inline asm on AMD

[Zhenzhong Duan]

On 2018/9/18 17:50, Peter Zijlstra wrote:
> On Mon, Sep 17, 2018 at 10:17:30PM -0700, Zhenzhong Duan wrote:
> > Lfence is preferred than general retpoline on AMD, add this option
> > in C / inline asm just as the ASM code does.
> >
> > For x86_64, it still help to have minimal retpoline for kernel even
> > if gcc doesn't support it, change the inline asm for x86 so that it
> > could also be used by x86_64.
> > Add ANNOTATE_NOSPEC_ALTERNATIVE for i386 to avoid below warning:
> > "warning: objtool: .altinstr_replacement+0x10: unsupported
> > intra-function call"
> > "warning: objtool: If this is a retpoline, please patch it
> > in with alternatives and annotate it with ANNOTATE_NOSPEC_ALTERNATIVE."
>
> This Changelog is almost unreadable, please rewrite.
Sorry, I'll rewrite it.

> Reverse engineering the patch you add RETPOLINE_AMD support to the
> inline-asm CALL_NOSPEC so that they match the asm CALL_NOSPEC.
>
> > Signed-off-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
> > ---
> >   arch/x86/include/asm/nospec-branch.h |   23 ++++++++++++++++-------
> >   1 files changed, 16 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > index fd2a8c1..2d49eab 100644
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -170,21 +170,26 @@
> >    */
> >   # define CALL_NOSPEC						\
> >   	ANNOTATE_NOSPEC_ALTERNATIVE				\
> > -	ALTERNATIVE(						\
> > +	ALTERNATIVE_2(						\
> >   	ANNOTATE_RETPOLINE_SAFE					\
> >   	"call *%[thunk_target]\n",				\
> >   	"call __x86_indirect_thunk_%V[thunk_target]\n",		\
> > -	X86_FEATURE_RETPOLINE)
> > +	X86_FEATURE_RETPOLINE,					\
> > +	"lfence;\n"						\
> > +	ANNOTATE_RETPOLINE_SAFE					\
> > +	"call *%[thunk_target]\n",				\
> > +	X86_FEATURE_RETPOLINE_AMD)
> >   # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
>
> That's OK.
>
> >   
> > -#elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
> > +#elif defined(CONFIG_RETPOLINE)
>
> This doesn't make any sense..
This change is used for x86_64 to have minimal Retpoline support when 
CONFIG_RETPOLINE is defined but RETPOLINE isn't defined, or I missed 
something?

> >   /*
> >    * For i386 we use the original ret-equivalent retpoline, because
> >    * otherwise we'll run out of registers. We don't care about CET
> >    * here, anyway.
> >    */
> >   # define CALL_NOSPEC						\
> > -	ALTERNATIVE(						\
> > +	ANNOTATE_NOSPEC_ALTERNATIVE				\
> > +	ALTERNATIVE_2(						\
> >   	ANNOTATE_RETPOLINE_SAFE					\
> >   	"call *%[thunk_target]\n",				\
> >   	"       jmp    904f;\n"					\
> > @@ -194,12 +199,16 @@
> >   	"    	lfence;\n"					\
> >   	"       jmp    902b;\n"					\
> >   	"       .align 16\n"					\
> > -	"903:	addl   $4, %%esp;\n"				\
> > -	"       pushl  %[thunk_target];\n"			\
> > +	"903:	add   $4, %%" _ASM_SP ";\n"			\
> > +	"       push  %[thunk_target];\n"			\
>
> Yeah, don't do that.
This is the change for above reason.

Thanks
Zhenzhong