Re: [PATCH v3] x86/bugs: Add a separate config for each mitigation

[Breno Leitao <leitao@debian.org>]

On Wed, Jun 28, 2023 at 07:21:28AM -0700, leitao@debian.org wrote:
> From: Breno Leitao <leitao@debian.org>
> 
> Create an entry for each CPU mitigation under
> CONFIG_SPECULATION_MITIGATIONS. This allow users to enable or disable
> them at compilation time.
> 
> If a mitigation is disabled at compilation time, it could be enabled at
> runtime using kernel command line arguments.

I had a chat about this topic with Boris and Thomas at Kernel Recipes,
and I would like to summarize the current state, and get it moving
forward.

1) The hardware mitigations are half-way added to KCONFIG. I.e., half of
the hardware mitigations are specified under SPECULATION_MITIGATIONS,
but not all of them.
	* You can enabled/disabled just half of them at build time.

2) It is impossible to build a kernel with speculative mitigations
disabled.
	* The only way to disable the mitigations is at boot time,
	  using the "mitigations=off" boot parameter.


So, disabling SPECULATION_MITIGATIONS, will only disable the mitigations
that are under SPECULATION_MITIGATIONS. Other mitigations will continue
to be enabled by default. This is is misleading for the user.

Here are a few options moving forward:

1) Create one Kconfig entry per mitigation, so, the user can pick and
choose what to enable and disable. (Version 3 of this patch. May need a
re-spin due to the new mitigations being added.)

2) Keep the Kconfig entries as-is. Create a new Kconfig entry
(CPU_MITIGATIONS_DEFAULT_OFF?) to disable the mitigations by default,
similarly to the `mitigations=off` boot parameter (v1 of this patch)

3) Same as 2, but, reusing SPECULATION_MITIGATIONS instead of
creating a new Kconfig entry.

4) Remove the current entries in SPECULATION_MITIGATIONS and the fine
control on what to enable/disable?!

What is the preferred way?