Re: [v2 1/3] x86/microcode/intel: Check microcode revision before updating sibling threads

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

On Thu, Feb 22, 2018 at 01:15:06PM +0100, Borislav Petkov wrote:
> On Thu, Feb 22, 2018 at 03:55:54AM -0800, Raj, Ashok wrote:
> > The current code wasn't trying to enforce checking the loaded microcode revision on a thread
> > before attempting to load the microcode. While you comeback from resume, if C0T0 already
> > is up, and we loaded the early microcode, then when handling C0T1 there is no need to 
> > do a wrmsrl to reapply microcode since its already loaded as part of C0T0. 
> 
> And I'm asking exactly this: is it simply "we don't need to do WRMSR" or
> "we should not"?
> 
> Because avoiding the WRMSR costs more than simply doing it and letting
> the HT thread ignore the supplied microcode.

This isn't a simple WRMSR like others. Microcode engine needs to do 
a bunch of validation.

> 
> If it is "we don't need to but there's nothing wrong when we do it" then
> we don't need this patch. And I'm pretty sure "nothing wrong when we do
> it" would be the answer. Otherwise we have bigger problems.

In the past the only guidance was to not load microcode at the same time to the 
thread siblings of a core. We now have new guidance that the sibling must be 
spinning and not doing other things that can introduce instability around loading 
microcode.

I think its safer to not load when its not required vs forcing a load and depending
on the microcode interface to not interfere. If the rules change in future we don't
have to adapt again.