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

[Len Brown]

On Sat, Mar 20, 2021 at 4:57 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:

> We won't enable features which are unknown ever. Keep that presilicon
> test gunk where it belongs: In the Intel poison cabinet along with the
> rest of the code which nobody ever want's to see.

I agree, it would be irresponsible to enable unvalidated features by default,
and pre-silicon "test gunk" should be kept out of the upstream kernel.

This patch series is intended solely to enable fully validated
hardware features,
with product quality kernel support.

The reason that the actual AMX feature isn't mentioned until the 16th
patch in this series
is because all of the patches before it are generic state save/restore patches,
that are not actually specific to AMX.

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).

There will be a future CPUID attribute that will help us identify
future simple-state features.
For AMX, of course, we simply know.

So after the generic state management support, the kernel enabling of AMX
is not actually required to run applications.  Just like when a new instruction
is added that re-uses existing state -- the application or library can check
CPUID and just use it.  It is a formality (perhaps an obsolete one), that
we add every feature flag to /proc/cpuid for the "benefit" of userspace.

The reason we propose this cmdline switch is
1. Ability of customers to disable a feature right away if an issue is found.
Unlike the CPUid cmdline that works on flags, this is the ability to turn
off a feature based on its state number.  Ie.  There could be 20 features
that use the same state, and you can turn them all off at once this way.

2. Ability of customers to enable a feature that is disabled by default
in their kernel.  Yes, this will taint their kernel (thanks Andy),
but we have customers that want to run the new feature on day 0
before they have got a distro update to change the default, and this
gives them a way to do that.

Yeah, the cmdline syntax is not a user-friendly mnemonic, and I don't know
that making it so would be an improvement.
Like the CPUID cmdline, it is precise, it is future-proof, and it is
used only in special situations.

thanks,
Len Brown, Intel Open Source Technology Center