Re: [PATCH v2 0/3] Freezer, CPU hotplug, x86 Microcode: Fix task freezing failures

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 10/11/2011 12:04 AM, Borislav Petkov wrote:
> Hi Tejun,
> 
> On Mon, Oct 10, 2011 at 02:08:48PM -0400, tj@kernel.org wrote:
>> Maybe I'm confused but is that patch correct for actual CPU hotplug
>> case?  If not, what's the point in doing that?  What are we gonna do
>> after six month some people come up with "CPU hotplug fails to load
>> new microcode for the new CPU"?
> 
[...]
> 
>> If somebody is sure that microcode don't need to be changed once
>> loaded, then all's good and dandy but that's not the case here, right?
> 
> Well, basically the current situation didn't change the ucode - it
> simply reloaded the same image from before going offline.
> 
> See, there's this another problem with what we have right now: imagine
> you've just updated the ucode image on disk and offline only a subset of
> the cores. Then you online them again and they now get the newer ucode
> image while the others still run the old ucode. This could explode or
> could not, one thing's for sure: all bets are off. If we don't reload it
> on hotplug, we're fine - only module reload triggers the ucode update in
> a fairly synchronized manner.
> 

This one makes a lot of sense to me. I hadn't thought of it before.

>> If you want to optimize away microcode unloading during
>> suspend/resume, the RTTD is doing revalidation / reload during
>> CPU_ONLINE as necessary.
> 
> see above.
> 
>> If this use case doesn't really matter too much to anyone, just
>> leaving it alone would be better than adding band aid which can lead
>> to very obscure issues down the road (oooh, that microcode shouldn't
>> have been loaded to that cpu).
> 
> I'd like to actually hear someone justify such a requirement.
> 
> I hope I'm making some sense here.
> 


-- 
Regards,
Srivatsa S. Bhat  <srivatsa.bhat@linux.vnet.ibm.com>
Linux Technology Center,
IBM India Systems and Technology Lab