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


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
  • Date: Tue, 26 Jan 2021 11:06:17 +0000
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  • Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>, "andrew.cooper3@xxxxxxxxxx" <andrew.cooper3@xxxxxxxxxx>, "george.dunlap@xxxxxxxxxx" <george.dunlap@xxxxxxxxxx>, "iwj@xxxxxxxxxxxxxx" <iwj@xxxxxxxxxxxxxx>, "julien@xxxxxxx" <julien@xxxxxxx>, "wl@xxxxxxx" <wl@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Tue, 26 Jan 2021 11:06:37 +0000
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  • Thread-topic: [PATCH v4 1/2] xen: EXPERT clean-up and introduce UNSUPPORTED

Hi,

> On 26 Jan 2021, at 09:22, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> 
> On 25.01.2021 22:27, Stefano Stabellini wrote:
>> A recent thread [1] has exposed a couple of issues with our current way
>> of handling EXPERT.
>> 
>> 1) It is not obvious that "Configure standard Xen features (expert
>> users)" is actually the famous EXPERT we keep talking about on xen-devel
>> 
>> 2) It is not obvious when we need to enable EXPERT to get a specific
>> feature
>> 
>> In particular if you want to enable ACPI support so that you can boot
>> Xen on an ACPI platform, you have to enable EXPERT first. But searching
>> through the kconfig menu it is really not clear (type '/' and "ACPI"):
>> nothing in the description tells you that you need to enable EXPERT to
>> get the option.
>> 
>> So this patch makes things easier by doing two things:
>> 
>> - introduce a new kconfig option UNSUPPORTED which is clearly to enable
>>  UNSUPPORTED features as defined by SUPPORT.md
>> 
>> - change EXPERT options to UNSUPPORTED where it makes sense: keep
>>  depending on EXPERT for features made for experts
>> 
>> - tag unsupported features by adding (UNSUPPORTED) to the one-line
>>  description
>> 
>> - clarify the EXPERT one-line description
>> 
>> [1] https://marc.info/?l=xen-devel&m=160333101228981
>> 
>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>

Answering to Jan...

> 
> Non-Arm bits
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> However, I have questions on the Arm ones (sorry for not noticing
> earlier, as I assume it was this way before already):
> 
>> @@ -77,7 +77,7 @@ config SBSA_VUART_CONSOLE
>>        SBSA Generic UART implements a subset of ARM PL011 UART.
>> 
>> 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.
>> 
>> config HARDEN_BRANCH_PREDICTOR
>> -    bool "Harden the branch predictor against aliasing attacks" if EXPERT
>> +    bool "Harden the branch predictor against aliasing attacks 
>> (UNSUPPORTED)" if UNSUPPORTED
>>      default y
>>      help
>>        Speculation attacks against some high-performance processors rely on
> 
> Both of these default to y and have their _prompt_
> conditional upon EXPERT. Which means only an expert can turn them
> _off_. Which doesn't make it look like these are unsupported? Or
> if anything, turning them off is unsupported?

...You could see that as EXPERT/UNSUPPORTED options can only be
 “modified” from their default value if EXPERT/UNSUPPORTED is activated.
So I find the current solution ok.

If this is a problem we could also change those options to be default to _off_ 
by renaming them to config DISABLE_xxxx

Cheers
Bertrand

> 
> Jan


 


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