Re: [PATCH v7 18/21] x86/clearcpuid: Support feature flag string in kernel option clearcpuid

From: Thomas Gleixner
Date: Wed Apr 17 2019 - 19:19:56 EST

On Wed, 17 Apr 2019, Fenghua Yu wrote:

> The kernel option clearcpuid currently only takes feature bit which
> can be changed from kernel to kernel.
> Extend clearcpuid to use cap flag string, which is defined in
> x86_cap_flags[] and won't be changed from kernel to kernel.
> And user can easily get the cap flag string from /proc/cpuinfo.

If your machine dies because init triggers #AC then please explain how that
easily can be read from /proc/cpuinfo and how the sysadmin can figure out
what the heck he needs to write on the kernel command line.

The whole 'clearcpuid' thing should have never been merged. It's a pure
testing/debugging thing. And no, we are not going to proliferate it and
extend it for dubious value. Quite the contrary, we should simply rip it

Add a simple 'noac' or whatever command line option, which is documented
proper and can easily be mapped to a #AC crash during boot.