Re: [PATCH] x86/retpoline: Fix NOSPEC_JMP for tip

[Tom Lendacky]

On 1/9/2018 6:40 PM, Thomas Gleixner wrote:
> On Tue, 9 Jan 2018, Linus Torvalds wrote:
> 
>> On Tue, Jan 9, 2018 at 4:31 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
>>>
>>> The following patch fixes it for me. Something doesn't
>>> seem to work with ALTERNATIVE_2. It adds only a few bytes
>>> more code, so seems acceptable.
>>
>> Ugh. It's kind of stupid, though.
>>
>> Why is the code sequence not simply:
>>
>>   ALTERNATIVE "", "lfence", X86_FEATURE_RETPOLINE_AMD
>>   ALTERNATIVE __stringify(jmp *\reg), __stringify(RETPOLINE_JMP \reg),
>> X86_FEATURE_RETPOLINE
>>
>> ie make that X86_FEATURE_RETPOLINE_AMD _only_ emit the "lfence", and
>> simply fall through to what will be the "jmp *\reg" of the
>> non-RETPOLINE version.
>>
>> Then just make sure X86_FEATURE_RETPOLINE_AMD disables X86_FEATURE_RETPOLINE.

I think there are areas that rely on X86_FEATURE_RETPOLINE being set
even if X86_FEATURE_RETPOLINE_AMD is set.  For example, line 261 in
arch/x86/entry/entry_32.S is only checking for X86_FEATURE_RETPOLINE.

Thanks,
Tom

>>
>> That is both simpler an dsmaller, no?
> 
> Duh, yes.
>