Re: [PATCH] x86/microcode: Add an option to reload microcode even if revision is unchanged

[Johannes Erdfelt <johannes@erdfelt.com>]

On Fri, Sep 06, 2019, Thomas Gleixner <tglx@linutronix.de> wrote:
> What your customers are asking for is a receipe for disaster. They can
> check the safety of late loading forever, it will not magically become safe
> because they do so.
> 
> If you want late loading, then the whole approach needs to be reworked from
> ground up. You need to make sure that all CPUs are in a safe state,
> i.e. where switching of CPU feature bits of all sorts can be done with the
> guarantee that no CPU will return to the wrong code path after coming out
> of safe state and that any kernel internal state which depends on the
> previous set of CPU feature bits has been mopped up and switched over
> before CPUs are released.

You say that switching of CPU feature bits is problematic, but adding
new features should result only in a warning ("x86/CPU: CPU features
have changed after loading microcode, but might not take effect.").

Removing a CPU feature bit could be problematic. Other than HLE being
removed on Haswell (which the kernel shouldn't use anyway), have there
been any other cases?

I ask because we have successfully used late microcode loading on tens
of thousands of hosts. I'm a bit worried to see that there is a push to
remove a feature that we currently rely on.

JE