Re: [PATCH v4 22/22] x86/fpu/xstate: Introduce boot-parameters to control state component support

[Dave Hansen]

On 3/25/21 3:59 PM, Len Brown wrote:
> We call AMX a "simple state feature" -- it actually requires NO KERNEL ENABLING
> above the generic state save/restore to fully support userspace AMX
> applications.
> 
> While not all ISA extensions can be simple state features, we do expect
> future features to share this trait, and so we want to be sure that it is simple
> to update the kernel to turn those features on (and when necessary, off).

>From some IRC chats with Thomaas and Andy, I think it's safe to say that
they're not comfortable blindly enabling even our "simple features".  I
think we're going to need at least some additional architecture to get
us to a point where everyone will be comfortable.

For instance, AMX might be "simple", but there are really only kludgy
ways to get it back to the init state.  Plus, it's *not* simple in that
state left in the registers can have permanent (as long as the state
remains) power and performance impact.

Also, we probably need to expand the "simple" architecture documentation
a bit.  For instance, we need to promise that things like pkeys which
can cause kernel exceptions will never be enumerated as "simple".