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

[Raj, Ashok]

Hi Thomas,

On Tue, Sep 17, 2019 at 08:37:10AM +0200, Thomas Gleixner wrote:
> > microode updates should be of 3 types.
> > 
> > - Only loadable from BIOS (Only via FIT tables)
> > - Suitable for early load (things that take cpuid bits for e.g.)
> > - Suitable for late-load. (Where no cpuid bits should change etc).
> > 
> > Today the way we load after a stop_machine() all threads in the system are
> > held hostage until all the cores have done the update. The thread sibling
> > is also in the rendezvous loop. 
> 
> I know. See below.
>  
> > Do you think we still have that risk with a sibling thread? 
> > (Assuming future ucodes don't do weird things like what happened in 
> > that case where a cpuid was removed via an update)
> 
> Well, yes. The sibling executes a limited set of instructions in a loop,
> but it might be hit by an NMI or MCE which executes even more instructions.

There is a plan to solve the NMI issue. Although there is one case we might
be showing as a spurious that might not be nice. If #MCE's showup there is
nothing we can do at that point. These are most likely unrecoverable. 
But we want to make sure we could atleast follow through with a proper reset.

Let me gather my thoughts on that when i have the patch ready to handle
those senarios.

> 
> So what happens if the ucode update "fixes" one of the executed
> instructions on the fly? Is that guaranteed to be safe? There is nothing
> which says so.
> 
> A decade ago I experimented with putting the spinning CPUs into MWAIT,
> which caused havoc. Did neither have time nor the stomach to dig into that
> further, but the ucode update _did_ fix an issue with MWAIT according to
> the version history.

Excellent point. 
> 
> That's why I'm worried about instructions being "fixed" which are executed
> in parallel on the sibling.
> 
> An authorative statement vs. that would be appreciated. Preferrably in form
> of an extension of the SDM, but an upfront statement in this thread would
> be a good start.

I have started the conversation internally. Once we have something solid
I'll share in the list, and also follow up with updates to SDM. 

Cheers,
Ashok