Re: [PATCH v2 22/22] x86/fpu/xstate: Introduce boot-parameters for control some state component support

[Andy Lutomirski]

On Thu, Nov 19, 2020 at 3:37 PM Chang S. Bae <chang.seok.bae@xxxxxxxxx> wrote:
>
> Rename XFEATURE_MASK_USER_SUPPORTED to XFEATURE_MASK_USER_ENABLED to
> literally align with new boot-parameters.
>
> "xstate.disable=0x60000" will disable AMX on a system that has AMX compiled
> into XFEATURE_MASK_USER_ENABLED.
>
> "xstate.enable=0x60000" will enable AMX on a system that does NOT have AMX
> compiled into XFEATURE_MASK_USER_ENABLED (assuming the kernel is new enough
> to support this feature).
>

What's the purpose of xstate.enable?  I can't really imagine it's
useful for AMX.  I suppose it could be useful for hypothetical
post-AMX features, but that sounds extremely dangerous.  Intel has
changed its strategy so many times on XSTATE extensibility that I find
it quite hard to believe that supporting unknown states is wise.