Re: [Xen-devel] [PATCH] x86/pv: Deprecate support for paging out the LDT

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> 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.


Jan



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