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

On Thu, Mar 25, 2021 at 7:10 PM Dave Hansen [off-list ref] wrote:

On 3/25/21 3:59 PM, Len Brown wrote:

quoted

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.

Hi Dave,

There is no code in this patch series, including patch 22, that enables
an unvalidated feature by default.

Yes, I fully accept that patch 22 allows a user to enable something
that a distro didn't validate.

If there is a new requirement that the kernel cmdline not allow anything
that a distro didn't explicitly validate, then about 99.9% of the kernel cmdline
options that exist today would need to be removed.

Does such a requirement exist, or does it not?

thanks,
-Len