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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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 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).

The code changes themselves look fine to me.

Jan



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