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Re: [Xen-devel] [PATCH for-4.13 2/2] xen/nospec: Introduce CONFIG_SPECULATIVE_BRANCH_HARDEN and disable it


  • To: Julien Grall <julien.grall@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 1 Oct 2019 10:41:44 +0100
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Norbert Manthey <nmanthey@xxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Tue, 01 Oct 2019 09:42:00 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 01/10/2019 10:26, Julien Grall wrote:
> Hi Jan,
>
> On 01/10/2019 10:22, Jan Beulich wrote:
>> On 01.10.2019 11:17, Julien Grall wrote:
>>> On 01/10/2019 00:21, Andrew Cooper wrote:
>>>> On 30/09/2019 21:17, Julien Grall wrote:
>>>>> My worry is this gate config gate nothing on Arm so the user may have
>>>>> a false sense that it can be used (even though it is clearly BROKEN).
>>>>>
>>>>> Also the name is quite close to the CONFIG_HARDEN_PREDICTOR on Arm
>>>>> and
>>>>> may confuse user. Although, I don't have a better name so far :/
>>>>
>>>> The "depends on BROKEN" means it will never show up to a user in
>>>> menuconfig, which is why it was only CC to x86, and not to rest.
>>>
>>> What's the long term plan for this option? Are you planning to
>>> remove it
>>> completely or just the dependencies on BROKEN?
>>>
>>> My concern is if this option will ever become selectable then it
>>> will not doing
>>> what's expected on Arm.
>>>
>>> So, even if in principle it is arch agnostic, it feels to me this
>>> option should
>>> better be implemented in x86/Kconfig.
>>
>> I don't think so, no. When BROKEN is to be removed, it ought to be
>> replaced by a suitable "depends on HAVE_*", which Arm could choose
>> to not select.
>
> So there are an expectation this option will be used by common code in
> the future?

It already is.  ARM has stubs for evaluate_nospec() etc.

My gut feeling is that the only way this is going to be resolved sanely
is with a compiler feature or plugin, at which point it reasonably can
be cross-arch.

~Andrew

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