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

[Zhenzhong Duan <zhenzhong.duan@oracle.com>]

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@oracle.com>
>> ---
>>   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