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

From: Raj, Ashok
Date: Fri Sep 06 2019 - 12:55:29 EST

Hi Thomas,

On Fri, Sep 06, 2019 at 02:51:17PM +0200, Thomas Gleixner wrote:
> Raj,
> On Thu, 5 Sep 2019, Raj, Ashok wrote:
> > On Thu, Sep 05, 2019 at 11:22:31PM +0200, Thomas Gleixner wrote:
> > > That's all nice, but what it the general use case for this outside of Intel's
> > > microcode development and testing?
> > >
> > > We all know that late microcode loading has severe limitations and we
> > > really don't want to proliferate that further if not absolutely required
> >
> > Several customers have asked this to check the safety of late loads. They want
> > to validate in production setup prior to rolling late-load to all production systems.
> Groan. Late loading _IS_ broken by definition and it was so forever.

Lets tighten the seat belts :-).. I'm with you that late-loading has
shown weakness more recently than earlier. There are several obvious reasons
that you are well aware. But there is a lot that *must* be done to make sure
the guard rails are tight enough for deplopying late-load. 100% agree on that
to make sure the interface and mechanism needs to be improved for robustness
but not a candidate for removal. Certainly this is an argument that would help
me drive towards that objective internally.

> 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.
> That does not exist and unless it does, late loading is just going to cause
> trouble nothing else.
> So, no. We are not merging something which is known to be broken and then
> we have to deal with the subtle fallout and the bug reports forever. Not to

When we did the late-load changes last year we added a warning if any
of the cpuid bits either dissappear or new ones appear. Maybe we should
have tainted the kernel to track that so its not that subtle anymore.

> talk about having to fend of half baken duct tape patches which try to glue
> things together.
> The only sensible patch for that is to remove any trace of late loading
> crappola once and forever.