Re: [PATCH v2 3/3] x86/Kconfig: introduce option to select retpoline usage

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.02.2022 11:34, Roger Pau Monné wrote:
> On Thu, Feb 17, 2022 at 10:07:32AM +0100, Jan Beulich wrote:
>> On 16.02.2022 17:21, Roger Pau Monne wrote:
>>> Add a new Kconfig option under the "Speculative hardening" section
>>> that allows selecting whether to enable retpoline. This depends on the
>>> underlying compiler having retpoline support.
>>>
>>> Requested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>> There's one aspect though which I would like to see Arm maintainer
>> input on:
>>
>>> --- a/xen/arch/x86/Kconfig
>>> +++ b/xen/arch/x86/Kconfig
>>> @@ -38,10 +38,6 @@ config GCC_INDIRECT_THUNK
>>>  config CLANG_INDIRECT_THUNK
>>>     def_bool $(cc-option,-mretpoline-external-thunk)
>>>  
>>> -config INDIRECT_THUNK
>>> -   def_bool y
>>> -   depends on GCC_INDIRECT_THUNK || CLANG_INDIRECT_THUNK
>>
>> Moving this ...
>>
>>> --- a/xen/common/Kconfig
>>> +++ b/xen/common/Kconfig
>>> @@ -146,6 +146,22 @@ config SPECULATIVE_HARDEN_GUEST_ACCESS
>>>  
>>>       If unsure, say Y.
>>>  
>>> +config INDIRECT_THUNK
>>> +   bool "Speculative Branch Target Injection Protection"
>>> +   depends on X86 && (GCC_INDIRECT_THUNK || CLANG_INDIRECT_THUNK)
>>
>> ... here despite being explicitly marked x86-specific looks a
>> little odd. Since the dependencies are x86-specific, dropping
>> X86 from here would make my slight concern go away.
> 
> Right - I've added the X86 because I was concerned about GCC or CLANG
> also exposing the repoline options on Arm, but that's not an issue
> because the compiler tests are only done for x86 anyway.
> 
> Feel free to drop the 'X86 &&' and the parentheses if you wish.
> Otherwise I can resend if you prefer.

No need to resend just for this.

Jan