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

["Raj, Ashok" <ashok.raj@intel.com>]

On Fri, Sep 06, 2019 at 11:16:00PM +0200, Thomas Gleixner wrote:
> 
> So if we want to do late microcode loading in a sane way then there are
> only a few options and none of them exist today:
> 
>  1) Micro-code contains a description of CPUID bits which are going to be
>     exposed after the load. Then the kernel can sanity check whether this
>     changes anything relevant or not. If there is a relevant change it can
>     reject the load and tell the admin that a reboot is required.

This is pretty much what we had in mind when we suggested to the uCode teams.

Just a process of providing a meta data file to accompany every uCode release.

IMO new cpuid bits are probably less harmful than old ones dissappearing.



> 
>  2) Rework CPUID feature handling so that it can reevaluate and reconfigure
>     the running system safely. There are a lot of things you need for that:
> 
>     A) Introduce a safe state for CPUs to reach which guarantees that none
>        of the CPUs will return from that state via a code path which
>        depends on previous state and might now go the other route with data
>        on the stack which only fits the previous configuration.
> 
>     B) Make all the cpufeature thingies run time switchable. That means
>        that you need to keep quite some code around which is currently init
>        only. That also means that you have to provide backout code for
>        things which set up data corresponding to cpu feature bits and so
>        forth.
> 
> So #2 might be finished in about 20 years from now with the result that
> some of the code pathes might simply still have a

Maybe we can catch the kernel side in 20 years.. user space would still be 
busted, or have a fault way to control new cpuid much like how we do for
VM's. 

> 
>      if (cpufeature_changed())
>      	   panic();
> 
> because there are things which you cannot back out. So the only sane
> solution is to panic. Which is not a solution as it would be much more sane
> to prevent late loading upfront and force people to reboot proper.
> 
> Now #1 is actually a sensible and feasible solution which can be pulled off
> in a reasonably short time frame, avoids all the bound to be ugly and
> failure laden attempts of fixing late loading completely and provides a
> usable and safe solution for joe user, jack admin and the super experts at
> big-cloud corporate.
> 
> That is not requiring any new format of microcode payload, as this can be
> nicely done as a metadata package which comes with the microcode
> payload. So you get the following backwards compatible states:
> 
>   Kernel  metadata	  result
> 
>   old	  don't care	  refuse late load
> 
>   new	  No   		  refuse late load
> 
>   new	  Yes		  decide based on metadata
> 
> Thoughts?

This is 100% in line with what we proposed... 

Cheers,
Ashok