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Re: [PATCH v4 1/2] xen: EXPERT clean-up and introduce UNSUPPORTED

  • To: Julien Grall <julien@xxxxxxx>
  • From: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
  • Date: Tue, 26 Jan 2021 14:23:28 +0000
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  • Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>, "andrew.cooper3@xxxxxxxxxx" <andrew.cooper3@xxxxxxxxxx>, "george.dunlap@xxxxxxxxxx" <george.dunlap@xxxxxxxxxx>, "iwj@xxxxxxxxxxxxxx" <iwj@xxxxxxxxxxxxxx>, "jbeulich@xxxxxxxx" <jbeulich@xxxxxxxx>, "wl@xxxxxxx" <wl@xxxxxxx>
  • Delivery-date: Tue, 26 Jan 2021 14:24:21 +0000
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  • Thread-topic: [PATCH v4 1/2] xen: EXPERT clean-up and introduce UNSUPPORTED

Hi Julien,

> On 26 Jan 2021, at 13:51, Julien Grall <julien@xxxxxxx> wrote:
> Hi Stefano,
> On 25/01/2021 21:27, Stefano Stabellini wrote:
>>  config ARM_SSBD
>> -    bool "Speculative Store Bypass Disable" if EXPERT
>> +    bool "Speculative Store Bypass Disable (UNSUPPORTED)" if UNSUPPORTED
>>      depends on HAS_ALTERNATIVE
>>      default y
>>      help
>> @@ -87,7 +87,7 @@ config ARM_SSBD
>>        If unsure, say Y.
>> -    bool "Harden the branch predictor against aliasing attacks" if EXPERT
>> +    bool "Harden the branch predictor against aliasing attacks 
>>      default y
>>      help
>>        Speculation attacks against some high-performance processors rely on
> I read through the back and forth between Bertrand and Jan about 
> "UNSUPPORTED". However, I still don't understand why those two options are 
> moved to UNSUPPORTED.

Discussion was more on what to do for options which have a default y and can 
only be turned off with UNSUPPORTED or EXPERT selected.

> Both options will only build the code to enable the mitigation. The decision 
> is still based on the processor you are running on.
> In addition to that, ARM_SSBD can also be forced enabled/disabled on the 
> command line.
> A user may want to compile out the code if the target processor is not the 
> affected by the two issues. This wouldn't be much different to Xen deciding 
> to not enabling the mitigation.
> I would view the two options as supported but not security supported. So this 
> seems to fit exactly in the definition of EXPERT rather than UNSUPPORTED.

I think you are right here, not security supported should be only available to 


> Cheers,
> -- 
> Julien Grall



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