Re: [Xen-devel] [PATCH 2/2] x86/microcode: Do not upload microcode if CPUs are offline

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.03.18 at 08:59, <chao.gao@xxxxxxxxx> wrote:
> This patch is to backport the patch below from linux kernel.
> 
>     commit 30ec26da9967d0d785abc24073129a34c3211777
>     Author: Ashok Raj <ashok.raj@xxxxxxxxx>
>     Date:   Wed Feb 28 11:28:43 2018 +0100
> 
>         x86/microcode: Do not upload microcode if CPUs are offline
> 
>         Avoid loading microcode if any of the CPUs are offline, and issue a
>         warning. Having different microcode revisions on the system at any 
> time
>         is outright dangerous.

I'm afraid I don't fully agree - not applying an ucode update to the online
CPUs because some are offline isn't any better. Plus (while updating)
there's always going to be some discrepancy between ucode versions.
As long as we apply updates while bringing a CPU online, I think we're fine.

Also please consider valid cases you make not work anymore, like someone
having brought offline all sibling hyperthreads, with each core still having
one thread active. In that case an ucode update will implicitly update all
offline threads as well.

Jan



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