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Re: [Xen-devel] [PATCH] x86/pv: Deprecate support for paging out the LDT
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 25 Jul 2018 13:18:28 +0100
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- Cc: Lars Kurth <lars.kurth@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxx, Julien Grall <julien.grall@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 25 Jul 2018 12:18:33 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 25/07/18 13:16, Jan Beulich wrote:
>>>> On 25.07.18 at 14:08, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 29/06/18 06:27, Jan Beulich wrote:
>>>>>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 06/28/18 6:10 PM >>>
>>>> On 28/06/18 14:35, Jan Beulich wrote:
>>>>>>>> On 26.06.18 at 13:35, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>>> --- a/xen/arch/x86/Kconfig
>>>>>> +++ b/xen/arch/x86/Kconfig
>>>>>> @@ -161,3 +161,24 @@ endmenu
>>>>>> source "common/Kconfig"
>>>>>>
>>>>>> source "drivers/Kconfig"
>>>>>> +
>>>>>> +menu "Deprecated Functionality"
>>>>>> +
>>>>>> +config LEGACY_PV_LDT_PAGING
>>>>>> + def_bool n
>>>>>> + prompt "PV LDT Paging-out support"
>>>>>> + ---help---
>>>>>> + For a very long time, the PV ABI has included the ability to
>>>>>> page
>>>>>> + out the LDT by transitioning its mapping to not-present. This
>>>>>> + functionality is believed to only exist for the PV Windows XP
>>>>>> port
>>>>>> + which never came to anything.
>>>>>> +
>>>>>> + The implementation contains a vCPU scalability limitation in a
>>>>>> + position which is prohibitively complicated to resolve. As
>>>>>> the
>>>>>> + feature is believed to be unused in practice, removing the
>>>>>> feature
>>>>>> + is the easiest remediation.
>>>>>> +
>>>>>> + If you discover a usecase which is broken by this option
>>>>>> being off,
>>>>>> + please contact xen-devel@xxxxxxxxxxxxxxxxxxxx urgently.
>>>>>> Baring
>>>>>> + something unexpected, the code and this option will be
>>>>>> removed.
>>>>> I think it should be said here explicitly when (or to be precise, no
>>>>> earlier
>>>>> than when) this is going to happen.
>>>> I'm open to suggests, but decided not to name a specific release (if
>>>> only to avoid second-guessing our future numbering and release schedule).
>>>>
>>>>> I also think the security support status with the option enabled needs to
>>>>> be clarified. Perhaps we'd go in stages: Introduce the (default off)
>>>>> option,
>>>>> then (e.g. for 4.13) switch its use to security unsupported, and finally
>>>>> drop the code (e.g. for 4.14).
>>>> I presume you mean that we should hide it behind EXPERT at that point?
>>> That's the best way to express it I guess, yes. Plus some form of remark in
>>> SUPPORT.md.
>>>
>>>
>>>> What does the middle step gets us. If its going to be off by default
>>>> and unable to be enabled by default, that is as good as deleted.
>>> Think of people only using released code: They'd notice the removed
>>> functionality only in 4.12. Removing the code right away for 4.13 could
>>> therefore be too early.
>> How can we unblock this patch? I stand by my original point of "that's
>> as good as deleted and isn't actually useful for distros".
> I don't see how the patch is blocked - I've clearly outlined how I think
> this (or in fact any such) removal should be done.
... which I disagree with and thing should be done in the way presented
in this path.
Hence the blockage.
~Andrew
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